Kingsley was born in New York City in October of 1906. He studied at Cornell University, where he began his career writing plays for the college dramatic club. He joined the Group Theater as an actor upon its formation. In 1933 the company performed his play Men in White. Set in a hospital, the play dealt chiefly with the morality of abortion, and was a box-office smash. It won the Pulitzer Prize for Drama in 1934.
Kingsley followed this success with the play Dead End in 1935. A story about slum housing and its connection to crime, the piece was also fairly successful, eventually spawning the Dead End Kids. The two plays which followed, the anti-war Ten Million Ghosts of 1936 and The World We Make of 1939, were flops and had short runs. But in 1943 Kingsley returned to his previous success with the historical drama The Patriots. This play, which told the story of Thomas Jefferson and his activities in the young American republic, won the New York Film Critics Circle Awards for Drama. Kingsley continued writing for the theater late into his career, adapting Arthur Koestler's novel Darkness at Noon for the stage in 1951, and writing Lunatics and Lovers in 1954 and Night Life in 1962.
In addition to his work for the stage, Kingsley wrote a number of scripts for Hollywood productions, mostly based on his own work.
His marriage to actress Madge Evans in 1939 lasted until her death in 1981.