Cassidy was already playing in bands at the age of 14 and, after a stint in the Navy during World War II, started playing in a line of groups from various genres. By the 50's, he had mainly settled into jazz playing with artists like Gerry Mulligan, Cannonball Adderly and Roland Kirk.
In 1964, he formed the band Rising Sons with Taj Mahal, Ry Cooder, Gary Marker (Captain Beefheart) and Jesse Lee Kincaid. He was part of the group for only a short time before he broke his hand and was replaced by Kevin Kelley (the Byrds).
A year later, he joined his stepson Randy California, Jay Ferguson and Mark Andes in the band Red Rooster. They later added John Locke and, in 1967, renamed themselves Spirit. Their first album, Spirit (1968), was a mix of psychedelia and jazz with the heavy use of string arrangements. It went to number 31 on the charts.
They followed with the soundtrack to Model Shop, which wasn't officially released until 2005, Clear (1969) and Twelve Dreams of Dr. Sardonicus (1970) before Cassidy left the group. He recorded with Randy California on his first solo album, Kapt. Kopter and the (Fabulous) Twirly Birds (1972) which was followed by The Adventures of Kaptain Kopter and Commander Cassidy in Potato Land (shelved by the record company).
In 1974, Cassidy an California reformed Spirit and they recorded the double album Spirit of '76, Son of Spirit and Farther Along. It all fell apart during the tour for the last album when California had a misunderstanding with tourmate Neil Young when he joined them on stage, pushing him away. The disagreement caused a rift in the group that was not able to be repaired.
During the 80's, the group reformed a couple of times for the albums Spirit of '84 (1984), Rapture in the Chambers (1988), Tent of Miracles (1990) and California Blues (1996). California died in 1997 in a surfing accident permanently ending the group
Cassidy went on to play with Merrell Fankhauser. He also dabbled in acting, most notably in the films 2010 and The Escape Artist and on TV's General Hospital along with pursuing his interest in history and writing.