Dennis Crosby attended Bellarmine College Preparatory in San Jose with his other brothers where he graduated in 1952 and subsequently enrolled at Washington State University at Pullman on an animal husbandry course with his twin brother. He dropped out of the course after two years and was inducted into the Army in January 1955. He was initially posted to Fort Ord, California and then transferred to Fort Riley in Kansas for training with the US 10th Infantry Division which transferred for duty in West Germany in the summer of 1955.