Evanier chose to be a writer after witnessing the misery his father felt from working for the Internal Revenue Service and contrasting that with the portrayal of a writer's life on The Dick Van Dyke Show.
He made his first professional sale in 1969 and almost immediately after that was taken on as an assistant to the legendary Jack Kirby. While apprenticing under Kirby, Evanier began writing foreign comic books for The Walt Disney Company. From there he was brought in to write stories for the American Disney Comics, Gold Key Comics, and comics for the Edgar Rice Burroughs estate.
In 1974 he teamed with writer Dennis Palumbo, since young comedy writers tended to come in pairs, and wrote for a number of television series including The Nancy Walker Show, The McLean Stevenson Show, and Welcome Back, Kotter.
After the cancellation of Kotter, on which he was one of the story editors, Evanier and Palumbo amicably ended their partnership. Evanier subsequently wrote for the Hanna-Barbera comic book division, a number of variety shows and specials, and began writing for animated cartoon shows including: Scooby Doo, Plastic Man, Thundarr the Barbarian, The ABC Weekend Special, Richie Rich, The Wuzzles, Dungeons & Dragons and Garfield and Friends.
He has also produced a number of comic books including Blackhawk, Crossfire and Hollywood Superstars (with Dan Spiegle), Groo the Wanderer (with Sergio Aragonés), and The DNAgents (with Will Meugniot).
He has been nominated for the Emmy Awards numerous times for his writing, but has yet to receive one.