The full name of the noble protagonist in Kate and Leopold is revealed to be Leopold Alexis Elijah Walker Gareth Thomas Mountbatten. The cop he is informing of this is not amused.
In Texas Across The River, Alain Delon's character introduces himself as Don Andrea Baldazar, El Duce de la Casala. Dean Martin's character replies, "And what do folks in a hurry call you?" before deciding to dub him "Baldy".
"How do you do Mrs. Leach? I'm, uh, Harvey. Manfred... jen... sen... den. I'm with the CIA."
Mrs. Leach not only takes this in stride, but indignantly addresses him as "Mr. Manfredjensenden" later on.
In Rodgers & Hammerstein's Cinderella, the Prince (who's giving a ball) is His Royal Highness, Christopher Rupert Windemere Vladimir Karl Alexander François Reginald Lancelot Herman Gregory James, son of Her Majesty, Queen Constantina Charlotte Ermintrude Guinevere Maisie Marguerite Anne and His Majesty, King Maximilian Godfrey Ladislaus Leopold Sidney Frederick John.
Abraham Delacey Giuseppe Casey Thomas O'Malley from Disney's The Aristocats.
Disney's Lady and the Tramp gave us the name of Pedro's sister; Rosita Chiquita Juanita Chihuahua.
Another Disney chihuahua: Ignacio Alonzo Julio Federico de Tito from Oliver & Company
Somewhat subverted in the remake of Mr. Deeds. Winona Ryder's character (lying) says she is from Winchestertonfieldville, Iowa. Later in the film, Adam Sandler's titular character somehow finds (and takes her to) Winchestertonfieldville, Iowa.
The classic "Laurel and Hardy" short The Music Box features Professor Theodore von Schwarzenhoffen, M.D., A.D, D.D.S.,F.L.D, F.F.F unt F. Thus combining this trope with Overly Long Title.
General Pompilio Montezuma de la Villa del Conde del Ombu, the comic relief sidekick in Alfred Hitchcock's Secret Agent - played by Peter Lorre, who has surprisingly good comedic timing (especially when delivering the name).
A mild example from Muppet Treasure Island: The full name of one of the crewmembers on the Hispaniola was Big Fat Ugly Bug-Faced Baby-Eating O'Brien. Don't ask.
President Dwayne Elizondo Mountain Dew Herbert Camacho from Idiocracy.
It's not a character name, but in Iron Man 1, people always respond to the Strategic Homeland Intervention, Enforcement, and Logistics Division with "Wow, you should shorten that." They do by the end of the film, to SHIELD.
Spy Kids: Carmen's full name is Carmen Elizabeth Juanita Echo Sky Brava Cortez.
Her brother's full name? Juni Rocket Racer Rebaldo Cortez.