Cartoonist and comic strip artist Jack Mendelsohn grew up in Brooklyn. His father was Winsor McCay's film agent and one of Mendelsohn's idols was local cartoonist Stan MacGovern. A high school dropout and Navy enlistee, Mendelsohn began his career in comics as a freelance gag cartoonist for magazines like the Saturday Evening Post after World War II. He was a productive scriptwriter for funny animal, humor and fantasy comic books by Quality, Dell and DC, as well as the syndicated 'Felix the Cat' strip (originally launched by Otto Messmer and Pat Sullivan). Mendelsohn moved to Mexico in 1951 and spent most of the decade there.
In 1959, he began his partially autobiographical comic 'Jacky's Diary', which he not only wrote, but also drew in a deliberately naïve, child-like style. His pastiche was so well done that many readers were fooled and thought it was created by an actual child. Even though Mendelsohn deliberately mentioned his age in the byline ("Jacky's Diary, by Jack Mendelsohn, age 32½") some parents still assumed the comic was some kind of contest for children and send in comics drawn by their own offspring, drawn in the style of a child. The strip ran from 1959 to 1961 and also had a comic book publication.
Mendelsohn moved in to television scriptwriting for Jay Ward's studio, as well as Hanna-Barbera's 'Scooby Doo' and the animated versions of Mort Walker's 'Beetle Bailey' and George Herriman's 'Krazy Kat'. He also co-wrote storylines for the ABC TV cartoon series 'The Beatles' (1965-1969) and was one of the co-screenwriters of the Beatles' animated feature 'Yellow Submarine' (1968), directed by George Dunning and with designs by Heinz Edelmann.
He was a ghost-writer for other comics artists, among several who published in the magazine Panic. In 2004 Mendelsohn received a Lifetime Achievement Award from the Animation Writers Caucus. Ten years later he received the Bill Finger Award for Excellence in Comic Book Writing.