Mastnak started in Imst in January 2008 for the first time in the European Cup and finished 71st in the parallel slalom . He made his debut in the World Cup in March 2011 in Chiesa in Valmalenco , which he finished in 39th place in the parallel giant slalom. In the same month he won there the gold medal in the parallel giant slalom at the Junior World Championships and finished fourth in the parallel slalom. At the 2011 Winter Universiade in Erzurum , he came fourth in the parallel giant slalom. In December 2013 he won the bronze medal in the parallel giant slalom at the Winter Universiade in Monte Bondone . The following month he reached his first top ten placement in the World Cup with fourth place in the parallel giant slalom in Rogla . At the Snowboard World Championships 2015 in Lachtal he came in 30th place in the parallel giant slalom and 28th place in the parallel slalom and at the Snowboard World Championships 2017 in Sierra Nevada on 39th place in the parallel giant slalom and 32nd place Rank in the parallel slalom. In the 2017/18 season, he finished in the top ten four times in 12 World Cup appearances. He came on the podium for the first time in the World Cup in Kayseri with third place in the parallel giant slalom and won his first World Cup victory in the parallel giant slalom in Scuol . At the highlight of the season, the 2018 Winter Olympics in Pyeongchang , he came 16th in the parallel giant slalom. At the end of the season he won his first European Cup victory in the parallel giant slalom in Rogla and finally achieved 11th place in the parallel world cup and ninth place in the parallel giant slalom world cup.
In the following season Mastnak won in Carezza and in the Secret Garden Skiresort and thus won the parallel giant slalom World Cup. He also finished second in the Parallel World Cup. At the Snowboard World Championships 2019 in Park City he won the silver medal in the parallel giant slalom.
Slovenia
World cup
Scuol
Carezza
Secret Garden Ski Resort