Thaddeus M. Mahon was born in rural Green Village, Pennsylvania. 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. He died in Scotland, Pennsylvania, in 1916. Interment was in Cedar Grove Cemetery in Chambersburg.