He served in the U.S. Navy during World War II, and after his discharge held many jobs before becoming a full-time musician again.
During this period, Cassidy performed together with many leading jazz musicians including Art Pepper, Cannonball Adderley, Roland Kirk, Lee Konitz and Gerry Mulligan.
After that, he formed the Red Roosters in 1965, with his fourteen year-old stepson Randy California, Jay Ferguson and Mark Andes.
Because California stepped out of the group for a period of time in 1972, following the death of Jimi Hendrix, Cassidy was the only member who played with all various line-ups of Spirit on almost 20 albums over almost 30 years.
Cassidy wrote, studied history, and continued to correspond with fans from his residence in Southern California until his death.