 He has an elder sister, Cecilia, and had a younger sister, Mary, who died in 1973.He attended the School of Music in Chatham Row to learn piano at the age of eight and then began drumming in 1971 at the age of 9, under the instruction of Irish drummer Joe Bonnie.After Bonnie's death, his daughter Monica took over for him, but Mullen gave up the lessons and started playing by himself.