He was born in Roemore, Breaffy, County Mayo, to primary school teacher parents, Thomas and Mary Mullen. He was educated at St. Jarlath's College, Tuam, and University College Galway. He too became a teacher and taught in Tullamore and North Brunswick St and at St Saviour's, Denmark St, in Dublin.
Mullen became active in the Irish Republican Army after the 1916 Easter Rising. He was the organiser of the escape from Rath Camp in the Curragh Camp during the Irish War of Independence. During the Irish Civil War, he was leader of the Tintown No 1 internment camp of 61 prisoners.
Mullen died in St Mary's Hospital, Phoenix Park. His last residence was 92 St Assam's Ave, Raheny, Dublin. He was survived by his wife, Louie (née Ryan), and three daughters. He is buried at St. Fintan's Cemetery, Sutton.