His comic strip The Nebbishes, largely forgotten now, was a huge hit in the 1950s and a mainstay of office wall decorations.
He is best known for his 1962 play A Thousand Clowns, which ran for two years. He received an Oscar nomination for he screenplay for the successful 1965 movie adaptation. The play was revived in 1996 and 2001. Both the 1962 play and the movie starred Jason Robards, Jr as Murray Burns, a charming, out-of-work writer with Peter Pan syndrome. He is forced to choose between social conformity and the probably loss of custody of his eleven-year-old son.
Gardner's biggest commercial success was the 1985 play I'm Not Rappaport, which ran for two years, won the Tony award for "Best Play," and became a 1996 movie.
Other plays included The Goodbye People (1968), Thieves (1974), and Conversations with my Father (1992),
He was screenwriter and co-producer of the 1971 motion picture "Who Is Harry Kellerman, and Why Is He Saying Those Terrible Things About Me?" which starred Dustin Hoffman.
He made a brief screen appearance as Rabbi Pierce in the 1987 motion picture Ishtar.