Cunningham was a member of the United States Army that reached the Island in the 1950s with the hydrogen bomb Jughead. ("Jughead")
He may have been killed on Richard Alpert's orders along with the rest of the U.S. Army contingent, whereupon his uniform was worn by an unnamed Other during the flaming arrow attack. ("The Lie") Alternatively, he may have successfully defected to the Others prior to their purge of the Army soliders, explaining why young Charles Widmore referred to the Other wearing his uniform as "Cunningham" when reporting to Alpert. ("Jughead")
He was later revealed to be one of Jacob's candidates. ("The Substitute")