Charles MacMahon was 11 years old when he took part in the 1916 Rising in Dublin as a member of Na Fianna Éireann. He was involved in carrying food and despatches between Irish Volunteer garrisons at the GPO, the Manure Works in Fairview, and Amiens Street Railway Station.

During the War of Independence he carried out intelligence work, as well as participating in IRA attacks on British forces/targets in Dublin in 1921 at Whitehall, Ballybough, Drumcondra Road, Killester, Binns Bridge, the London and North Western Hotel and the attack on the Custom House.

He was wounded in the head during the attack on the Custom House; the bullet was not removed until 1925. Charles MacMahon joined the National Army in March 1922 and took part in the attack on anti-Treaty IRA forces in the Four Courts at the outbreak of the Civil War. However, due to ongoing medical difficulties arising from the wound received by him in 1921, MacMahon saw little further active service and was discharged medically unfit in January 1924.