Gaynor joined Simple Minds in 1982 as a session drummer for the New Gold Dream album (as a recommendation by the record producer, Pete Walsh), playing on six of the nine tracks. He later joined the band permanently for the New Gold Dream tour, as a replacement for Mike Ogletree. Except for a period (1992-97) away from the band after the Real Life tour of 1991-92, he has been the band's drummer to the present day.
Gaynor began drumming at age 11 and had his first professional engagement at age 14.
In addition to drumming, he is active as a songwriter and a record producer. Currently, he has also been working on a side project as part of a duo (along with Stephane Deriau-reine) called The Fusion Project. The Fusion Project's first album was released in April 2006 and is described as "a mix between progressive rock and jazz-fusion."
Mel was also a member of Birmingham-based 'Muscles', a funk-oriented covers band that had minor chart success with "If it Relaxes Your Mind" and "I'm a Girlwatcher."
Gaynor has played a variety of drum sets over the years, including Yamaha and currently Ludwig. He has played a variety of cymbals as well, including at first Zildjian during the first years of Simple Minds, but then switched to Paiste in the early 1980s, which he played for the duration of his career. He has also endorsed Anatolian and UFIP cymbals as well, but in 2011, Paiste announced him as an official endorser again, and has remained so to this day.