In 1976 he recorded his first single - Morning Girl which hit the top ten in Holland. He followed this with the theme for the television movie Dawn: Portrait Of A Teenage Runaway. His next single, That's Rock 'N' Roll, was a top 10 smash in Australia and Germany.

Further successful singles followed, including the 1977 Number 1 US smash Da Do Ron Ron.

Cementing his status as a teen idol, he co-starred with Parker Stevenson and Pamela Sue Martin in 1978's The Hardy Boys/Nancy Drew Mysteries television series. He appeared in a number of guest spots culminating in his own show - The Shaun Cassidy Special. He was also nominated for a Grammy Award as Best New Talent - In 1993 he made his Broadway debut (with his brother David) in Blood Brothers.

He later focussed on writing and producing where he worked on numerous productions including Cold Case and The Agency and was creator of the cult television series American Gothic.