Müllenberg won the silver medal at both the 2013 European Amateur Boxing Championships and 2015 European Amateur Boxing Championships. At the 2016 European Boxing Olympic Qualification Tournament in April 2016 he qualified for the 2016 Summer Olympics after beating Mehmet Ünal in the semi final. He is the first Dutch boxer since 1992 who qualifies for the Summer Olympics. He was eliminated in the second bout at the Olympics.