Cassidy's first wife was actress Kay Lenz, whom he married in 1977 and divorced in 1982. According to their friend, Sandie Clark, David and Kay were best friends before they married and it didn't work out. His second wife was South African sportswoman Meryl Tanz, whom he married in 1984. This marriage ended in 1985. On March 30, 1991, Cassidy married Sue Shifrin-Cassidy, by whom he has a son, Beau Devin Cassidy. He also has a daughter, actress Katie Cassidy (born in 1986), from his relationship with 70s model Sherry Benedon.

The Globe reported that Cassidy once slept with his Partridge Family co-star Susan Dey. He said he loved the teenage actress like a sister when they were shooting The Partridge Family but that she was (unbeknownst to Cassidy) in love with him. When the show ended, Dey confided her feelings and that couple slept together once, but Cassidy regretted it. He told The Globe "I find a certain sluttiness very attractive in a woman, and Susan just didn't have it.