Leonard was born at Brookly. Several of Leonard's plays reached Broadway, most notably A Majority of One in 1959. He also wrote Climax and Playhouse 90 for television in addition to scripting eleven Academy Award shows.

He authored a number of books including his collaboration on Edward G. Robinson's autobiography. During WWII he served as a colonel in the Army Signal Corps and edited "Army, Navy Screen Magazine". He died in Los Angeles County, California.