Kassir played the Bulgarian kicker, Zagreb Shkenusky, for 7 years in the HBO comedy series 1st & Ten, about a fictional football team. His other live action credits include sketch-acting on The Amanda Show and a portrayal of Shemp Howard in a biopic about The Three Stooges, produced by Mel Gibson, and Ralph in Reefer Madness: The Movie Musical. He was also able to get his own Pee-wee Herman style show in 1997 called Johnnytime, which aired for two seasons on the USA network. He also appeared as The Atom in the Justice League of America pilot episode, which, despite not being picked up, was released as a film in some markets. John has appeared in over a dozen feature films, starred in eight TV pilots (six of which went to series and two of which lasted on air for over six years each) and guest starred in dozens of TV series in both comic and dramatic roles. Kassir beat out both Michael Winslow and Charles Fleischer to become the voice of the Crypt Keeper.
As a standup comic, he has opened for the likes of Lou Rawls, Tom Jones, The Temptations and Four Tops on their TNT tour, Bobby Vinton, Richard Belzer, and Harry Blackstone, Jr. among others. John has also appeared on stage performing improvisation with Robin Williams, though Williams did not realize this was happening.
More recently, John has specialized in voice-over work for animation, video games, and other productions. Notable in this regard has been his acting the roles of Ray "Raymundo" Rocket on Rocket Power. He had a lead role in a romantic comedy, Channels (2008 film). He also provided the voice in the video games X-Men Legends II: Rise of Apocalypse, Marvel: Ultimate Alliance and Marvel: Ultimate Alliance 2 for fan-favorite mercenary Deadpool, becoming the preferred voice actor, as well as Sauron and Pyro, two other well-known characters. He was also featured in the very first commercial for the Nintendo cornerstone video game series The Legend of Zelda in the United States in 1987.
A few of his many theater credits include: originating the role of Kenny in Three Guys Naked From The Waist Down (also starring Scott Bakula), for which he was nominated for both a Drama Desk and an Outer Critics Circle award. John also originated the role of Ralph in the musical Reefer Madness, on stage and in the film version.
He provided the voice of the mischievous raccoon Meeko in Disney's 1995 animated feature Pocahontas and Pocahontas II: Journey to a New World. Also, along with Charlie Adler, he provided the voice of Buster Bunny in four Tiny Toon Adventures cartoons (following Charles Adler's departure from the role). He played the villainous Scuttlebutt in An American Tail: The Treasure of Manhattan Island. He also made one voice appearance on Ben 10 as Zombozo in the episode, "Last Laugh." He voiced the character Adam MacIntyre and others in the video game Dead Rising. John is also featured in the cartoon short Adventure Time as the Fire Element and Ice King