One of his major achievements was the third place in winter Polish Championship in Zakopane in 2007, and the silver medal in summer Polish Championships in 2011. At the World Junior Championship took third place in team competition in Zakopane together with Maciej Kot, Łukasz Rutkowski and Dawid Kowal. In 2011, he won the qualification for the Lotos Poland Tour competition in Zakopane. He took part in World Junior Championships in Tarvisio (2007), Zakopane (2008), Hinterzarten (2010) and the Estonian Otepaa (2011). At the 2010 Winter Olympics, he finished 36th both in the individual large and normal hill events. Miętus's best World Cup finish was fifth in the team large hill event at Finland in 2009.