Shaun was born on September 27, 1958 and traveled with his parents when he was only six months old while they were doing shows. His parents were stage and screen actors Shirley Jones and Jack Cassidy whom were both heavily involved in the business. When he was a pre-teen, he started a band called "Every Mothers Dream" and when his mother and half-brother, David Cassidy, were both starring in a musical TV sitcom called The Partridge Family, he asked his mom to put him in some episodes. However, she refused as she wanted to encourage him to concentrate on his schooling. He did appear as part of the chorus in many musical theatre productions. In 1972, he then sang in a punk rock band called "The Longfellow" which was short-lived, as his parents transferred him to a private school in Beverly Hills.

He had a chance to star with his mother as James Preston in a summer stock stage musical production of "On A Clear Day You Can See Forever" to get a taste of acting. During his grad year, he decided to make some records in Germany and had a hit there called "Morning Girl" and appeared in TV shows with his hit there as well as in Australia. Then, in late 1976, he signed up with agent Ruth Aarons and played Christopher Wentworth Hewlitt in a short independent film called Born of Water and successfully auditioned for the role of Joe Hardy in The Hardy Boys Mysteries. Also he had a dream of becoming a writer and re-wrote some of the scripts of the series to make it look suitable for him which angered the producers of the show. On an episode of the show, he sang some numbers as his character in a local disco, which led to a number of hit 45s and albums.

In the 80s, he made acting a primarily focus after his role in the movie of the week titled Like Normal People (1979) (TV)_ which he was well credited in as the mentally challenged Roger Meyers as well as playing Dave Stohler in the short-lived series "Breaking Away" (1980)_ based on the popular motion picture which only 8 episodes were produced due to a writers strike. He then decided to learn the true craft of acting and pursued it on stage by starring in such productions like "Mass Appeal", "The Subject Was Roses", "Diary of a Hunger Strike", "Dangerous Music", "Pass/Fail" and "Blood Brothers" with Petula Clark and his half-brother David Cassidy - in their first show together. After a few more TV guest roles, movies of the week as well as acting on stage, he pursued his dreams as was mentioned and retired from acting to produce and write TV shows and films, including the short-lived but well-acclaimed American Gothic and well as the other TV series "The Agency" (2001) and Invasion. He also write the sequel to the hit motion picture Midnight Run titled Midnight Run for Your Life which became a noted TV movie. Writing and producing became Cassidy's most successful career in the industry today like for many other celebrities like former actor Ron Howard. He admitted to this day he was only half good as a singer and admitted he lad some limitations as an actor and felt he didn't enjoy it enough to pursue for the rest of his life.
Daughter, with Susan Diol, Juliet Jones Diol Cassidy, was born. [19 March 1998] Son of actress Shirley Jones and actor Jack Cassidy Brother of Patrick Cassidy and Ryan Cassidy; half-brother of actor David Cassidy His song, Da Do Ron Ron reached #17 on the German charts in April of 1977. This was the same year his half-brother's (David Cassidy) song Gettin' It On The Street reached #12 on the same exact chart. Is a vegetarian. Made his Broadway debut in Willy Russell's Blood Brothers with his half-brother David Cassidy in 1993. Children by ex-wife Ann Pennington: Caitlin Ann Cassidy (b. 25 November 1981) and John (Jake) Pennington Cassidy (II) (b. 27 February 1985). Middle name Paul comes from his maternal grandfather. Was born 1 month after his expected due date. He and his brothers were delivered via Cesarean section. Won a Dramalogue award for best actor in a stage production of "Diary of a Hunger Strike". Backed into a vehicle that was stopped at the gate of Warner Bros. Studios, pushing the vehicle into a third vehicle, pinning a security guard between the two vehicles. Cassidy was not cited and the case remains under investigation. (19 February 2004) Went to the same High School as Angelina Jolie, Michael Klesic, Nicolas Cage, David Schwimmer and Lenny Kravitz. Welcome the arrival of child #4, a boy named Caleb on March 3, 2005 Signed his first autograph at age 5. The reason? For being Shirley Jones' son. Welcomed child #5, a boy, named Roan Howard on September 23, 2006.