Standard Police Motto

The Los Angeles Police Department's famous motto, "to protect and to serve" (coined in 1955), has been adopted by many other police departments across the English-speaking world, sometimes in its alternate form "to serve and protect." It's been adopted by so many police departments, in fact, that it's practically a Stock Phrase in the English-speaking world. Unsurprisingly, it shows up a lot in fiction as well. Played straight in most Police Procedurals and Cop Shows, it's routinely subverted, lampshaded, and/or otherwise played with in other media.

Film
"Bob Morton: What are your Prime Directives?
 * Robo Cop:

Robo Cop: Serve the public trust, protect the innocent, uphold the law."

Literature

 * The Casseline Brotherhood, in Kushiels Legacy, say this a lot.

Live Action TV

 * The Trope Maker (in the media, at any rate) is Dragnet, which was based on the Real Life LAPD.
 * In Due South, when Constable Fraser is asked if the RCMP motto is We Always Get Our Man, he replies with the correct motto of Uphold the Right.

Tabletop RPGs

 * To Serve and Protect was the title of a Champions module.
 * Protect & Serve was the Cop Splatbook for Cyberpunk 2020.

Web Comics

 * Paradigm Shift uses this trope to Show Their Work: "We Serve and Protect," the motto of the Chicago police department, can be clearly read on the side of a Chicago police cruiser on this page.
 * This is Trenton's motto in Dead Winter. He's the last good cop in a corrupt police force.

Western Animation

 * In the "Sleeping Beauty" episode of the ALF Tales cartoon, it is said of the Three Nice Wizards (led by wizard Joe Froday) by the King: "They protect and serve."
 * Re Boot: "My format: Guardian - To mend and defend."
 * The motto comes up as a point of discussion between Goliath and Eliza during the Gargoyles pilot, during their first meeting. Probably goes a long way towards building the trust between them.

Film

 * In the Michael Bay Transformers film, the Decepticon Barricade has a police car mode with the phrase "To punish and enslave."
 * In the film The Mask, Stanley Ipkiss tries to get a policeman guarding him to help Tina (who's being chased by Mooks) by telling him "Serve and protect!"

Literature

 * Shows up several times in the Discworld series:
 * The Ankh Morpork Watch has the motto "Fabricati Diem, Pvnc", which is Dog Latin for "Make My Day, Punk". In-story, Fred Colon claims that it translates as "To Protect and Serve". The full motto is in fact "Fabricati Diem, Pvncti Agvnt Celeriter," but has been rendered partially unreadable over time. This supposedly means "Make the Day, the Moments Pass Quickly."
 * Pratchett also uses it in Guards Guards with the prophecy that Carrot fulfills: "Yea, the king will come ... and Protect and Serve the People with his Sword."
 * In Feet of Clay, the motto on the Vimes family arms (defunct since Old Stoneface killed the last king) is "Protego Et Servio". The Vimeses have been watchmen for a long time.
 * The same book has the  give his own spin on the Robo Cop example above: "To Serve The Public Trust, Protect The Innocent, And Seriously Prod Buttock".
 * In the Myth Adventures, Asprin uses this for one of his chapter-beginning quotes: "To Serve and Protect. Traditional motto of protection rackets."

Live Action TV

 * Mathnet's motto is "To cogitate and to solve."
 * In Corner Gas, Karen and Davis have a long, drawn-out argument over whether or not their cop car says "To Protect and Serve" or "To Serve and Protect."
 * They also decide to change it at one point, because Karen feels the standard motto is too common.
 * At least partially in a Babylon 5 episode where organic tech gets loose creating a warrior who shouts "Protect" before he blows large holes in people and the station.

Video Games

 * A police officer in Destroy All Humans can sometimes be heard constantly repeating this to themselves if their mind is read. Until they get it wrong: "swerve and defect...dammit, I lost it!"
 * In The Longest Journey, the same cop that gave us Move Along Nothing to See Here and Sorry, I'm Gay lines, also finished his conversation with "We are here to protect and to serve", right after proving as useless and outright obstructive to the heroine as he possible could.
 * The motto of the Los Santos Police Department appears to be "Obey & Survive".

Web Original

 * SCP Foundation: To Secure, Contain and Protect (hence the name of the Foundation). This one could plausibly go in the "played straight" column too.
 * In Sonichu the Jerkops ride patrols with the motto "To Shatter and Torture All Hearts".

Western Animation

 * In The Simpsons, an Itchy & Scratchy cartoon shows shows Scratchy entering a police car that turns out to be driven by Itchy. The motto it "To Protect and Sever". Bloody carnage ensues.
 * South Park: The side of Officer Barbrady's patrol car has "To Patronize and Annoy" on it.
 * The tagline of Science Court was "To Serve and Observe", even though it wasn't about a police department.