Guercio moved to Los Angeles in the mid-1960s and began working as a session musician and songwriter. He played on several recordings, wrote Chad & Jeremy's 1966 Top 30 pop hit "Distant Shores", and is listed as a "contributor" to Frank Zappa's 1966 debut album "Freak Out!" (he was briefly a member of the Mothers of Invention prior to the album's recording). Hired by Columbia Records as a staff producer, Guercio began working with the Buckinghams and helped them create four big 1967 hits including two Top Tens — "Don't You Care" and "Mercy, Mercy, Mercy" — and two singles which peaked immediately outside the Top Ten: "Hey Baby (They're Playing Our Song)" and "Susan".