In 1969, Holmes married childhood friend Elizabeth "Liza" Wood Dreifuss, an attorney. Holmes's daughter Wendy died suddenly in 1986, at the age of ten, of an undiagnosed brain tumor. He has two sons, Nick and Timothy, the latter of whom has autism.
In his 20s, Holmes was a session musician (producing sessions, writing and arranging songs, singing and playing a few instruments). In 1969, Holmes and Ron Dante (the Cuff Links, the Archies) recorded "Jennifer Tomkins" for release on their second album, The Cuff Links. During the recording of that album, Dante was prohibited by the studio that produced the Archies from any involvement in new recording ventures and was forced to drop out of The Cuff Links. Holmes finished the project and released "Jennifer Tomkins" separately under a different studio name, Street People (not related to the mid-1970s band of the same name). The song was on the Billboard Hot 100 for 15 weeks, beginning January 3, 1970, reaching a peak of 36. A follow-up single called "Thank You Girl" reached 96 on the Billboard pop charts in April 1970.