He was born in Beaver Dam, Wisconsin, and moved with his family to Orange County, California, when he was four. There, Hatfield played baseball and briefly considered going professional, but his passion for music led him to pursue music while in high school. He would eventually encounter his singing partner Bill Medley while attending California State University, Long Beach.
The pair began singing as a duo in 1962. Their first single was "Little Latin Lupe Lu"; their first hit was "You've Lost That Lovin' Feelin'", produced by Phil Spector in 1964. Follow-up hits included "(You're My) Soul and Inspiration" and "Unchained Melody". The duo broke up in 1968.