Kassir first became known in 1985, when he defeated Sinbad for best stand-up comic on Star Search.
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 also provided the voice for the mercenary Deadpool in the video games X-Men Legends II: Rise of Apocalypse, Marvel: Ultimate Alliance, and Marvel Ultimate Alliance 2, as well as for 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.
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, he provided the voice of Buster Bunny in four Tiny Toon Adventures cartoons (following Charlie Adler's departure from the role); in the same show, he also voiced Bugs Bunny in at least one episode. He played the villainous Scuttlebutt in An American Tail: The Treasure of Manhattan Island. He also made one voice appearance in an episode of Ben 10 as Zombozo. He voiced the character Adam MacIntyre and provided Additional voice over work in the video game Dead Rising. Recently, his voice is featured in the games Shadows of the Damned and Metal Gear Rising: Revengeance as Hell demons and Desperado Enforcement captain Monsoon, respectively.
He then 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 the 2000 TV biopic 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.
In 2011, he voiced Crazy Smurf in the live-action/animated film The Smurfs, and in 2012, he voiced Jealousy as well as Gumball, Penny, Leslie, Carrie & Darwin when he possesses them in The Amazing World of Gumball episode "The Flower". Kassir appeared in Bryan Singer's Jack the Giant Slayer (2013), along with Bill Nighy, as Fallon, the two-headed leader of the giants; Nighy played the big head while Kassir played the small head.
Kassir will return to his iconic Cryptkeeper role in 2019.
