 During the American Civil War, he enlisted in August 1862 as a private in Company A, One Hundred and Twenty-sixth Regiment of the Pennsylvania Volunteers.After a term of service in this regiment, he reenlisted as a veteran in January 1864 in the Twenty-first Regiment, Pennsylvania Volunteer Cavalry, and served until September 1865.Mahon studied law, was admitted to the bar in 1871, and commenced practice in southern Pennsylvania.