The song was written by P. F. Sloan, one of the better songwriters of the mid-Sixties. He died some years ago. It was Phil Sloan who played the opening guitar riff on “California Dreaming” by the Mamas and the Papas.
Mamas and Papas did backing vocals on Barry McGuire’s album before they recorded their first M&P album.
IIRC, McGuire introduced M&P to Lou Adler (Dunhill Records). Later, Adler would marry a rising young star...Shelly Fabares