Leo was born on March 27, 1928, in Brooklyn, New York, and enlisted in the United States Marine Corps in early 1946. He began his service at Parris Island before moving on to Cherry Point Naval Air Station in North Carolina, then to the Marine Barracks at Quantico, Virginia, where he attended the Amphibious Warfare School. His principal duty was as a general stockman and clerk. Leo earned the Sharpshooter designation and was discharged on November 10, 1947, holding the rank of Private First Class, from the Marine Casualties office at the Marine Barracks, Brooklyn Navy Yard, not far from the Brooklyn neighborhood where he grew up.

Though Leo did not serve overseas or see combat, his time in the Corps came during the immediate postwar years when the Marine Corps was reshaping itself for a new era, and the men who filled its ranks in those years were essential to that transition.

He was honorably discharged on November 10, 1947– the Marine Corps birthday.