Sam Roy Hagar was born to Bobby and Gladys Hagar in Salinas, California and was named after his maternal grandfather. Hagar's family worked in the lettuce fields and he lived with his parents and three siblings in a labor camp until moving south to Fontana, California. There, his father Bobby had landed a job at the Kaiser Steel Mill, working in the open hearth. Bobby Hagar was an alcoholic boxer who held a record for being knocked down 20 times in a single fight. Friends said he was "mentally damaged" upon returning home from fighting in World War II. "My father was the town drunk," said Hagar, who described his father as "a complete alcoholic and madman" who was abusive towards his wife and children. Hagar's mother would occasionally take the children to a nearby orange grove to sleep in the car when their father became too violent. The Hagar family moved frequently, as Bobby had a habit of spending the rent money on alcohol; Hagar recalls living in nine different homes in Fontana while growing up. When Hagar was ten years old, his mother Gladys took the children and left Bobby for good.