Mercer's start in the music industry was as a singer, but quickly moved to drums while still a teenager. He played several different styles in his early years, including Latin and jazz, before becoming a member of a Montreal-based band known as The Triangle, who later changed their name to Mashmakhan upon obtaining a record deal in Toronto. Mercer's work can be heard on the band's most successful single "As the Years Go By". As part of Mashmakhan, Mercer played on the Festival Express tour and was interviewed for the 2003 documentary.
Following Mashmakhan's dissolution in the early 1970s, Mercer played for Roy Buchanan, and then The Wackers, before joining April Wine, who were in the process of changing their line-up following the departure of two founding members in 1973. His first public performance with the band was as an opening slot for T. Rex and Three Dog Night at Toronto's Exhibition Stadium. Shortly thereafter he completed his first album with the band, Electric Jewels, which contained the hit "Weeping Widow". Mercer remained with the band consistently since joining, and played on all their subsequent albums and tours. In November 2008, he stated that he was retiring at the end of the year. He was replaced by Blair Mackay in January 2009. In May 2010, Mercer became the first Canadian to receive a Legends Award at the 10th annual Cape Breton International Drum Festival.
In addition to performing, Mercer was also involved in manufacturing drums with a company he co-founded with Bill Hibbs called Monolith Drums. Briefly assuming the brand name "Solstice Drums", now back to Monolith and about to celebrate 20 years of drum making. Mercer is no longer involved with Monolith Drums.