Leo Margulies was born in Brooklyn, New York, and raised in Norwalk, Connecticut. He attended Columbia University, but left before completing a degree. In 1932, he began working at the Munsey chain of pulp magazines. He left Munsey for Thrilling Publications, where he became editorial director of Thrilling Wonder Stories in 1936. He edited as many as 46 pulp magazines at a time, including Captain Future, Startling Stories, Fantastic Universe, and Strange Stories.
During World War II, Margulies served as a war correspondent. After the war, he began to edit and publish science fiction and fantasy paperback anthologies. In 1953, he started The Saint Detective Magazine with Leslie Charteris, following that up in 1956 with Mike Shayne's Mystery Magazine, where he remained as editor until his death.
