His father, Gabriel Heyward Mahon Sr., was a successful merchant who served as mayor of Greenville.

He attended The Citadel in Charleston, South Carolina for two years as a member of the Class of 1909 before withdrawing to begin a business career.

During the war, he served in the United States Army as a captain and later major in 1st Battalion, 118th Infantry regiment, 30th Division.

After the war, he was active in the American Legion, including service as South Carolina's state commander.

During World War II, he was executive director of South Carolina's Council of defense.