On 15 March 1935, he set a ski jumping world record at 95 metres (312 ft) on Bloudkova velikanka hill in Planica, Kingdom of Yugoslavia and gained international success and recognition.

His best chance came at the 1938 FIS Nordic World Ski Championships in Lahti where he struggled to win over the famous Ruud brothers: (Birger, Sigmund, and Asbjørn).

Ruud thought about handing the gold over to Marusarz during the medal ceremony, but he settled on naming him the "Moral World Champion".

He finished 27th in the 18 km cross country event, 22nd in the nordic combined and 17th in ski jumping at the 1932 Winter Olympics in Lake Placid, New York.

During World War II, Jan escorted famous Polish SOE agent Krystyna Skarbek across the Tatra Mountains, on skis, from Hungary into Poland.