Kolašinac was a finalist at the 2010 European Athletics Championships, where he placed ninth overall. He has competed for Serbia in indoor track and field events, including the 2010 IAAF World Indoor Championships and the 2011 European Athletics Indoor Championships. Outside of major competitions, he won silver medals for Serbia at the 2010 European Cup Winter Throwing meet and at the second league levels of the 2010 and 2011 European Team Championships.