Hernandez grew up in the San Francisco Bay Area suburb of Pacifica, California. He was a star athlete at Terra Nova High School in Pacifica before transferring to nearby Capuchino High School, from which he graduated in 1971. One of his teammates on the Terra Nova High School baseball team was future major league pitcher Bob McClure, who had also played Little League baseball with him when they were younger. Hernandez went on to the College of San Mateo, a local community college, and was drafted by the St. Louis Cardinals in the 42nd round of the 1971 amateur draft. He made his major league debut on August 30, 1974.