Seaver was born in Fresno, California, to Betty Lee (née Cline) and Charles Henry Seaver. He attended Fresno High School and was a pitcher for the school's baseball team. Seaver compensated for his lack of size and strength by developing great control on the mound. Despite being an All-City basketball player, he hoped to play baseball in college. He joined the U.S. Marine Corps Reserve on June 28, 1962. He served with AIRFMFPAC 29 Palms, California, through July 1963. After six months of active duty in the reserve, Seaver enrolled at Fresno City College. He remained a part-time member of the reserve until his eight-year commitment ended in 1970.