Born in Toronto, Canada, Simon Marcus was a troubled child, excelling in sports but getting expelled from school for fighting. At age 16, he discovered a nearby Muay Thai gym and began training there. After trying to pursue a basketball career and serving three months in a youth detention center, he returned to Muay Thai kickboxing with the wish to become a pro fighter.

Turning professional in 2009, Marcus began to compete all over the world in several promotions, including Lion Fight, where he won the Lion Fight Heavyweight Championship over future fellow GLORY star Artem Levin at Lion Fight 9 in March 2013. It was also in Lion Fight where he scored a huge win over Joe Schilling, a win Schilling would get revenge on about a year later at GLORY 17, Marcus’ first appearance at GLORY.

Marcus won the middleweight contender tournament at GLORY 20 and earned a title shot against Levin. With their first title bout going to a draw, Marcus won the rematch and title at GLORY 27: Chicago, in early 2016. After retaining the title over Dustin Jacoby, Marcus lost the title to Jason Wilnis at GLORY 33: New Jersey.

Marcus defeated Jacoby again at GLORY 34 before getting the title back from, and getting revenge on, Wilnis at GLORY 40: Copenhagen in April 2017.