Prior to becoming a full-time musician, Mercer worked both as a cattle farmer and as an IBM computer programmer.
With the departure of Trevor Payne the group became known as The Triangle,[5][6] who later changed their name to Mashmakhan upon obtaining a record deal in Toronto.
[7][8] Following Mashmakhan's dissolution in the early 1970s, Mercer played for Roy Buchanan,[9] and then The Wackers, before joining April Wine,[5][10] who were in the process of changing their line-up following the departure of two founding members in 1973.
[3][10] His first public performance with the band was as an opening slot for T. Rex and Three Dog Night at Toronto's Exhibition Stadium.
[12][13] In May 2010, Mercer became the first Canadian to receive a Legends Award at the 10th annual Cape Breton International Drum Festival.