Experienced Protagonist

"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects."

- Robert A. Heinlein

Most works have the protagonist be a Farm Boy, New Meat, an Ordinary High School Student or Rookie Red Ranger.

Well, This Is Not That Trope.

An Experienced Protagonist starts out the story already knowing what he's doing, whether through formal lessons or being Taught By Experience. He's done what he does for a while. He might be The Dreaded or Famed in Story or a Living Legend, or a mentor to a genuine newbie, or even a Retired Badass (or soon-to-be) doing One Last Job. Any account of his origins or early days will be shown in prequel Origins Episodes, flashbacks or discussed after the fact, if it is ever depicted onscreen at all.

Despite this, such a character is not necessarily exempt from Not Seeing That Coming, Mid-Season Upgrades, progression, and seeking out a mentor of his own. Perhaps he was a big fish in a small pond and the Big Bad is an Outside Context Villain who exposes him to a higher form of war that what he already knows and can do is So Last Season against. Perhaps some reason is contrived for him to Restart At Level One despite the initial A Taste of Power. Whatever the case, "continuous learning" can be as much a truism for the well-established fictional character as it is in reality.

Other than competence, past experience can also be a source of drama and plot points. Such a protagonist could have old enemies coming back for a second round, past misdeeds haunting him, That One Case weighing him down, My Greatest Failure to atone for, or the emotional scars of being a Shell-Shocked Veteran to name just a few. This kind of character may still have an Origins Episode as a Flashback or other kind of Prequel, but it won't be the starting point for the series.

Anime and Manga

 * Kenshin Himura of Rurouni Kenshin is already a war veteran, legendary assassin known as "Battōsai", and one of Japan's best swordsmen at series start. A number of antagonists come after him specifically because of his past deeds.
 * Kousei Arima from Your Lie in April is a Famed in Story former Child Prodigy pianist who stopped playing two years ago after a breakdown following the trauma of his mother's death. Much of the early part of the series revolves around him regaining the will to play and how others in the scene react to his return.
 * In contrast to the franchise's tendency to (outside of direct sequels) star characters who wind up in a mobile suit by accident or necessity or new meat that winds up way over their head, Domon of Mobile Fighter G Gundam is already recognized as one of the Earth sphere's greatest martial artists by the start. There's still difficulty because almost everyone else in the tournament is as well, and still room for character growth because Domon is only good at martial arts and has underdeveloped social skills.
 * In a rare overlap with Kid Hero, Garrod of After War Gundam X is already an experienced survivalist at the start, fully capable of capturing an enemy mobile suit with just some minor gadgets and a gun. Being born on the apocalypse and living in the post-apocalypse will do that. Garrod's growth is more about his transition from survivor to fighter and, like Domon, needing to work better with others.
 * Full Metal Panic!'s Sousuke Sagara is a Child Soldier and the series derives both comedy and drama from the conflict between the habits burnt into him by military experience and the expectations of civilian high school life.
 * Gate: Yōji Itami is already a certified Ranger and JSDF special forces when the Gate first opens.
 * Overlord: The protagonist is sent to a fantasy world while trapped in the guise of his VRMMO character Momonga, a Level Capped lich with equally top-tier items and servant NPCs, and accidentally sets what remains of his guild on a course for world domination.

Comic Books

 * If you value your life or your sanity, do not underestimate Morpheus aka Dream in The Sandman. He's been around since conscious minds learned to dream and Morpheus exists in multiple dimensions and forms. Calliope, the muse of poetry, reveals that he had a child with her, Orpheus. Sure the story starts with him being captured by a sorcerer, but it's explained as one chance in a million and Alexander Burgess was trying to catch Death but missed. Lucifer dislikes Dream personally, but is not foolish enough to challenge him in a direct fight; he has other means of defusing the Endless.
 * Ballister Blackheart in Nimona has been around the block enough to know when not to fight fair against the Institute, and how to evade their military. What Nimona brings to the table is more firepower and means infiltration, to turn the tide of the populace and show the Institute is corrupt.

Fan Works

 * In the Digimon fic The Teacher of All Things the protagonist is the Taichi "Tai" Yagami from Digimon V-Tamer 01 now being trown in the events of Digimon Adventure one year after his first incursion in the digiworld. Part of the drama of the fic is because Tai is experienced but the circumstances have changed (he is now with other children when he used to work solo, he has got a new digimon while still mourning his previous one, and the current crisis is so different to the one he previously solved he ends Wrong Genre Savvy) and he refuses to confess his previous experience to the other Digidestined for the longest time.
 * Doug Sangnoir of Drunkard's Walk began his extradimensional exile with ten years of experience as a paramilitary superhuman under his belt. In that time he saved or helped save his homeworld several times, created a martial arts style, taught young superhumans, reached a rank of Colonel, and eventually became the Security Chief of Warriors International, the superteam of which he was part all that time.
 * Taylor Hebert is this in the Worm/Harry Potter crossover A Wand for Skitter -- inserted into the body of a murdered 11-year-old Muggleborn girl at the end of her career as both a hero and a villain, she approaches the core conflict of the Wizarding World with all the skills, insight and experience she acquired in her years as a Parahuman.

Film

 * Dredd: The eponymous Judge is a veteran lawman who is tasked with evaluating newbie Anderson.
 * Bryan Mills from Taken has "a very particular set of skills; skills ... acquired over a very long career." The kidnappers of his daughter laugh off his attempt at getting them to let her go. They don't live to regret doing so.
 * Carl Fredericksen in Up is a senior citizen whose day job used to be selling balloons at the zoo. Nevertheless, he has the sense to pack for an ostensibly permanent trip to Paradise Falls and keeps Russell as safe as possible. They end up working together to save a giant bird named Kevin, with Carl realizing he can be badass, and Russell taking inspiration after seeing Carl in action. Realizing that he needs Russell in his life as much as the kid needs him,.
 * Agent Lance Sterling in Spies in Disguise runs into problems because of this trope. He is one of the best spies that the US has, but as a result, he is an egotistical ass that would rather blow up enemies first rather than at least analyze the situation for the best solution or wait for backup. People love him, but he doesn't have friends that will vouch for him when he gets framed. Most of the movie's problems with the disguise serum are that Lance steals what he thinks is Walter's drink before Walter can explain that it was actually the serum, and is still in the experimental phase. Sterling later admits to Walter, the gadgeteer genius he fired for creating peaceful inventions, that part of the reason is he's like that is that he's seen too many good people fall in the field and going solo and dirty means that he minimizes casualties..
 * Commando: John Matrix is a retired special forces colonel. Some bad guys make the mistake of kidnapping his daughter in an attempt to get him to do their bidding. They quickly learn why that was a bad idea.
 * Played for drama as much as badassery in the Rambo films. John Rambo is a Vietnam War veteran and very good at what he does, but also heavily emotionally scarred from his experiences.
 * The eponymous character of John Wick is one of the best assassins in the world, or at least he was before he completed an Impossible Task that helped The Mafiya gain control over New York in exchange for his retirement. Even his enemies and rivals speak of him with fear and respect.
 * Ip Man is already a respected Wing Chun master, albeit one who has refused repeated requests to take up students, by the start of the first film.
 * Top Gun: Maverick is already a capable aviator at the start of the film. TOPGUN is not a boot camp for complete nuggets; it exists to take already competent personnel and push them even further beyond.

Literature

 * Outside of his juvenile novels, Robert A. Heinlein strongly favored competent, experienced people for his heroes. Male or female, they were usually proficient in multiple fields of endeavor, experts in their specialties, and often well-versed in many other fields as well.
 * Even at the start of Casino Royale, James Bond is considered an experienced and capable agent.
 * Mack Bolan from The Executioner is an elite sniper and penetration specialist fighting in the Vietnam War. He learns that his father was being pressured by The Mafia, snapped and committed murder-suicide on the rest of his family, and decides to put his skills to good use fighting a war at home instead.
 * While the main protagonist of the Monster Hunter International series starts off new to the monster hunting thing, even if he's very good with guns and brawling, Alpha and Nemesis shift the prospective to his boss and the government's top anti-monster agent . As the two, they naturally qualify.
 * The Monster Hunter Memoirs spinoff stars Oliver Chadwick Gardenier who is both a marine's marine, proficient with every small arm in service, divinely blessed by God himself with an ability to naturally pick up even the most complicated languages, saved from death by an explicit miracle, and trained in a variety of useful talents before becoming a monster hunter and then breezed by his earliest assignments prior to the assignment the book is mostly about. The opening of each book and ending to the series have other characters present state the viewpoint character whose memoirs you're reading was fond of tooting his own horn and was absolutely exaggerating his own ability, even if the core of the events is true, but it's clear Chad had some experience before the events in Seattle.
 * At the start of Saga of the Forgotten Warrior, Ashok Vadal is not only one of Lok's Protector of the Law, making him a Judge Dredd-esque uncaring enforcer of the Law and a One Man Army, he's considered one of its best and credited with having revived the order that was sliding into extinction though how much of a paragon of it he is. How does this protagonist get challenged? By.
 * A Study in Scarlet has Sherlock Holmes already recognized as a detective worth employing, and Watson an accomplished but medically retired army surgeon.
 * Jack Reacher mustered out after over a decade of being a military policeman, and he puts the fighting and investigative skills developed over the course of his career to good use in the cases that come up.

Live-Action TV

 * Holt is a protagonist in Brooklyn Nine-Nine if not the protagonist – that goes to Jake Peralta – but he has enough experience in NYPD as an openly gay, black officer. The man is stern, literal-minded, doesn't seem to know how to tell a joke, and snarky, but he is wise and introspective. Jake even comes to see him as a father figure, accidentally calling him "Dad" while interrogating a perp. If you want to know how the 80s were, just ask Holt since he has stories about being mistaken for being a perp, founding the first NYPD organization, arresting a serial killer while mocking him Jake-style, and watching cigar-smoking friends succumb to lung cancer.
 * All three of the central trio in Burn Notice are experienced at their backgrounds, and Michael is even infamous enough to make Russian special forces uneasy. The only reason they're doing odd-jobs in Miami is that Michael has been victim of a grand conspiracy and is unable to get "normal" work.
 * Most investigators in the Law & Order franchise have been detectives for a while, with even replacement characters in the rotating cast typically being transfers or newly partnered instead of freshly promoted. This makes sense, since (with the exception of Law & Order: Special Victims Unit) the franchise works on Always Murder and stars homicide detectives. The prosecutor side is more willing to have relatively inexperienced prosecutors, but (appropriately) almost always sticks them in second chair and is also appropriate for the murder focus.

Newspaper Comics

 * Dr. Frank Melisch in Liberty Meadows may not be good with women, or socializing, but he is a competent veterinarian. A constant running gag is how he's one of the two sanest people on the preserve, reassuring Leslie about hypochondria bouts and tending to Ralph's various wounds. Likewise, Dr. Brandy Carter is so well-versed in a veterinary study that she even has failsafes designed for when Dean the pig falls back to cigarette usage.

Puppet Shows

 * Kermit the Frog would like you to know that he has a lot of experience running The Muppet Show. He's been a newscaster on Sesame Street, an MC for decades, and a charismatic personality The problem is that nothing ever goes to plan and no sufficient wisdom will fix that: whether it's Gonzo's stunts going awry, Wayne and Wanda's special effects messing up their numbers or a guest deciding to be a troll, you can expect chaos. Nevertheless, Kermit pushes forward, in the original Muppet Show and its subsequent spinoffs.

Video Games

 * Alpha Protocol: Four of the five default background choices for Mike Thorton have him with achievements and experience whether in the military, government service or as a globetrotting Freelancer that have drawn the attention of the eponymous agency. This experience is reflected in having higher base skill points right from chargen. The sole exception is "Recruit", but completing a run with that selected unlocks the Veteran background that takes this Up to Eleven with Mike being a member of a similar organization that sought Alpha Protocol out by himself. Accordingly, a Veteran Mike has even more starting points and some unique dialogue reflecting this greater experience.
 * Many Grand Theft Auto protagonists are current or former criminals.
 * The various player characters of the Halo series have been fighting the Covenant invasion - or, in the case of the Arbiter, fighting for them - for years to decades by the time of the first game they're playable in.
 * Commander Shepard of Mass Effect has already gone through The Spartan Way needed to become a N7 and experienced combat, including one of three incidents in which he or she played a prominent role, before the player takes control for the first time.
 * Martin Walker of Spec Ops: The Line starts as a captain in Delta Force. Not that it helps much against what's waiting for him and his in Dubai.
 * Kazuma Kiryu of the Yakuza series has already had a multi-year career as a gangster and renown as the "Dragon of Dojima" when the first game starts. Unfortunately, a 10-year stint in prison early on leaves him rusty and in need of regaining his strength.
 * Max Payne has already been a NYPD detective when control of him is first gained in the prologue of the first game. By the time the first chapter starts, he has had another three years of service as a DEA agent undercover with the mob.
 * Cyberpunk 2077: Whether as a well-evaluated Corpo field agent and middle manager, a Nomad from out of town, or a Streetkid who grew up dodging both the cops and gangs, in all Lifepaths V already knows how the world works, even if far from the dizzying heights that will be eventually reached.

Web Comics

 * The Chinese webmanhua Cheating Men Must Die stars Su Luxia, a quick transmigrator that by the beginning of the story has centuries of experience avenging mistreated side characters and crushing cheating scum of both sexes, and proceeds to keep doing so. Her first mission is actually told in a side story.

Western Animation

 * This is why in Samurai Jack, the Big Bad Aku wants to keep Jack from returning to the past. Jack is an experienced samurai well-versed in demonslaying and defending the innocent. He thus is a viable threat, since his sword is the only weapon that can kill Aku..