Mullen was born and raised in Dublin, and attended Mount Temple Comprehensive School, where he co-founded U2 in 1976 after posting a message on the school's notice board.
Mullen naceu e medrou en Dublín, e atendeu ao Mount Temple Comprehensive School, onde cofundou U2 en 1976 despois de deixar unha mensaxe no taboleiro de anuncios da escola.