Simeon Thoresen began his training in 2003 training in BJJ and Muay Thai. He would win his first and only amateur match in 2005 and would then start training under former Dream and Shooto champion Joachim Hansen. Thoresen would win his first 9 fights finishing all of his opponents and claiming the Adrenaline Sports Welterweight title. He would also go on to win the Ultimate Warrior Challenge Welterweight title which he would defend once before being signed to the UFC. He defeated Besam Yousef by rear naked choke in his UFC debut before being finished by Seth Baczynski in his second match for the company.