Almost concurrently, Cassidy starred in the ABC television series The Hardy Boys Mysteries, as well as Breaking Away and had a stint on the daytime soap General Hospital.

While appearing on Broadway in the hit musical drama Blood Brothers, he wrote his first television pilot, American Gothic.

He scored a couple of hit singles in several countries, leading to an American release of his first solo album, Shaun Cassidy, in 1977.

His popularity continued with the concurrent arrival of his television series, The Hardy Boys Mysteries (1977–1979), which also starred Parker Stevenson.

Cassidy's next album Born Late netted the hit "Hey Deanie" (#7), also written by Carmen, and a remake of The Lovin' Spoonful's "Do You Believe in Magic?"

He appeared on Broadway and West End productions such as Mass Appeal and Bus Stop, as well as the American premiere of Pass/Fail at the Mark Taper Forum in Los Angeles.

Cassidy won a Dramalogue award for his work in Mark Sheriden's Diary of a Hunger Strike at the Los Angeles Theater Center, and his final stage appearance was opposite his half-brother David in the Broadway production of Blood Brothers (which successfully ran for over a year on Broadway, landing Cassidy on the cover of “People” magazine for the third time).

[1] In 2021, Cassidy took his one-man show The Magic of a Midnight Sky to the stage, playing to standing-room-only crowds nationwide, his first live music performance in almost forty years.