Armstrong's musical career started while he was still a schoolboy, when he played in Belfast's top showband, The Melotones, who were resident in the city's Romano's Ballroom.
After leaving Them in 1969 Armstrong again teamed up with vocalist Kenny McDowell (who had replaced Van Morrison) in Sk'Boo, alongside Ricky McCutcheon and Colm Connolly.
Then he and McDowell (born Kenneth McDowell, 21 December 1944, in Belfast, Co Antrim) formed the band Truth (1969–71) in Chicago with bassist Curtis Bachman (ex The Buckinghams) and drummer Reno Smith (ex Baby Huey & the Babysitters).
In the late '70s, after playing with McDowell in Spike, Jim Armstrong was guitarist with Northern Irish rock outfit Light.
He is currently (2015) living in Las Vegas, having moved to the U.S. following a health glitch when touring with his Jim Armstrong Band in Germany.