Mikić won his last international cap for the Croatian under-19 team on 24 March 1999 and went to make his debut for the country's under-21 team in their opening match of the 1999 World Youth Championship in Nigeria, against Ghana on 4 April 1999. He appeared in all four matches played by the Croatian team at the tournament before they were eliminated by Brazil in the round of 16. Until the end of the year 1999, he made five appearances and scored three goals for the Croatian under-21 team in their qualifying campaign for the 2000 European Under-21 Championship and subsequently also appeared in all three group matches at the final tournament in Slovakia, where Croatia was eliminated in the group stage. He continued to play for the Croatian under-21 team until November 2001, making six appearances and scoring three goals in their qualifying campaign for the 2002 European Under-21 Championship.