Hammerstrom was born in Jamestown on December 30, 1917, and his sports career began in 1932 when he competed in the 220-yard hurdles for the then Jefferson Junior High School track team. Later that year, and through 1935, he was involved in the Jamestown High School football program.
Hammerstrom was a football letterman in 1934 and 1935, when he was the captain. He also competed in track in the 220-yard hurdles in 1933 and 1934. In 1935, Hammerstrom captained that 3-5-1 football squad under Coach Dent Moon. He led the Red Raiders in scoring and was outstanding at quarterback and running back.