Breen began appearing as a nightclub and stage singer at age seven, becoming nationally famous in 1936 when he became a regular on Eddie Cantor's weekly radio show. That same year he was hired by RKO and shortly was the studio's leading child star. Before his voice changed he appeared in several films as a singing actor, beginning with Let's Sing Again (1936). His last film was Johnny Doughboy (1942). After his film career he continued working as a singer in nightclubs and a musical performer in stock. He graduated from UCLA and was a guest pianist for the NBC Symphony Orchestra on radio. Breen served in the infantry in World War Two, then studied at the American Academy of Dramatic Arts. For a while he had a local TV show in New York.