The son of a German immigrant from Cologne and an Irish mother from Dublin, Charles Joseph Steinmayer joined the Irish Volunteers in 1915. During the 1916 Rising he served in the GPO and Moore Street and was interned until December 1916. Following his release, Charles Steinmayer was employed as a clerk in various positions, including with a British Army pay office in Dublin in 1918.

During the War of Independence he was arrested and interned between November 1920 and December 1921. He joined the National Army in September 1922 and served throughout the remainder of the Civil War. He resigned from the Defence Forces in January 1929.