De Leon, who original name was Gerardo Ilagan, is a member of the Ilagan clan of Philippine motion pictures. This clan produced movie thespians Robert Arevalo, Liberty Ilagan and Ronaldo Valdez; and musical scorer Tito Arevalo. De Leon was a medical doctor by profession, but his ultimate love for film has won him over. He made another 8 movies as an actor before he became a director.
Nicknamed "Manong" and considered as "the Master," De Leon holds the sole distinction of being the most awarded film director in the history of the Filipino Academy of Movie Arts and Sciences' FAMAS Awards, the Philippines' Oscars. From 1952 to 1971, he was awarded seven FAMAS Awards, three of them he received three years in a row. What was more astonishing was that all the films in which he won for Best Director also won for Best Picture at the FAMAS. He won for Sawa sa Lumang Simboryo (1952), Hanggang sa Dulo ng Daigdig (1958), Huwag Mo Akong Limutin (1960), Noli Me Tangere (1961), El Filibusterismo (1962), Daigdig ng mga Api (1965), and Lilet (1971).
He died on July 25, 1981.