James M. Knepper was born on November 22, 1927 in Los Angeles, California. He began his musical career with the trombone at the age of nine. He started playing professionally at the young age of 15, in jazz bands and at dance clubs in the 1940’s. Knepper’s first big job was with Freddie Slack, a jazz piano player leading his own big band. After that he worked with other jazz big bands led by Roy Porter, Charlie Spivak, Claude Thornhill, Woody Herman, and even Stan Kenton in 1959. He finally gained notoriety and attention while playing under Charles Mingus.