Born at Flower Fifth Avenue Hospital in New York, his ancestors were both immigrants and colonial Americans -- his Father, half German and half Irish, while his mother's ancestors were the founders of Newark, New Jersey. Throughout his early childhood, his parents were frequently touring and thus weren't around as often, instead spending most of his time raised by his maternal grandparents. At one stage during his childhood, his parents had divorced and made the decision not to tell him. It wasn't until several years later, when he'd heard the from a neighbor, that he realized what had happened. His parents argued that because they were often gone, his life would remain mostly unchanged and decided not to sour his emotional stability.

Eventually, Cassidy moved in with his father and Shirley Jones, his father's new wife. Together, they helped David pursue acting and theater further, with Jack introducing him to his first manager -- Ruth Aarons, a close friend of Jack's who would later prove to be a big force behind David's early success.

The broadway musical The Fig Leaves Are Falling served as David's debut as a well-known actor. Later that year, in 1969, he moved to Los Angeles where he signed with Universal Studios and was featured in various roles in television series like Bonanza and Marcus Welby, M.D. The very next year in 1970, one of David's first and most well known roles took shape as he began work on The Partridge Family. During season one, he initially was working on set for small wages, before those wages were renegotiated for much better ones from Aarons. Initially, he was cast aside as just a "looks" actor, before convincing the music producer that he could indeed sing, and was thus promoted to lead singer for the series' recordings. Around this time, he began work on his solo music career as well, releasing covers for The Association and The Young Rascals -- which, peculiarly, charted much higher in the U.K. than they did in the U.S.

By the late 70s, Cassidy took a turning point in his career, deciding to quit acting and focus primarily on recording alone. His success continued and his next several albums were all received with praise, with the hit "I Write the Songs" becoming the first recording artist hit.

Cassidy's life was sprinkled with various relationship challenges. His first marriage was with actress Kay Lenz on April 3, 1977. They found common ground because both David’s father had died in an apartment fire five months earier and Kay’s father also had recently just died. They had met one another during one the sets for “The Partridge Family”. Their relationship only lasted four years, and had a divorce in 1981. By 1984, David married his second wife, Meryl Tanz, a horse breeder from South Africa, whom he met when he planned on building a stud farm. She introduced Thoroughfare Horseracing to David. David’s farm was as short lived as his relationship with Meryl, and they were separated, after two years of marriage.

He also had an affair with fashion model Sherry Benedon for almost 20 years that ended in 1990. Katherine Evelyn “Katie” Cassidy, a famous actress from “Gossip Girl”, is David’s daughter from his relationship with Benedon, although they were never married. David had a bad relationship with his daughter. Katie claimed that David was shutting her out of his life and abused her verbally. Benedon added the David was not paying his share for Katie’s hospital bills and was not providing enough child support for Katie, although David’s camp claimed that his relationship to his daughter was only getting affected because Benedon still feels resentment for their broken relationship.