Foster was born on April 17, 1939, in San Mateo, California. He attended elementary and high schools there, and was a member of the varsity football and track teams. After graduating from high school in 1957, he went to work as an automobile mechanic helper. He enlisted in the Marine Corps Reserve on November 4, 1961, in San Francisco, California, and received recruit training with the 1st Recruit Training Battalion at Marine Corps Recruit Depot San Diego, and individual combat training with the 2nd Infantry Training Regiment at Marine Corps Base Camp Pendleton.