Erwin started on trumpet at age four. He played in several territory bands before joining the groups of Joe Haymes (1931–33) and Isham Jones (1933–34). He then moved to New York City, where he was prolific as a studio musician, performing on radio and in recording sessions. He played with Benny Goodman in 1934-35, then with Ray Noble in 1935; the next year he joined Goodman again, taking Bunny Berigan's empty chair. In 1937 he again followed Berigan, this time in Tommy Dorsey's orchestra, where he remained until 1939.