He is the father of singer/songwriter Kali Armstrong and the younger brother of musician Rick Armstrong, a member of the progressive rock band Edison's Children.
He wrote the song "Flight of Fancy", the end-credit song of the documentary film "Armstrong". He also sings backup vocals on the track.
Marillion guitarist Steve Rothery mentioned in an interview that Mark Armstrong had once played with them in a soundcheck and his daughter is called Kayleigh, named after the band's biggest hit song (from 1985).