Arendz, from Springton, P.E.I., was clocked in 43:11.2 in the men's 15-kilometre standing race, and missed just one shot in four rounds of shooting.
"It was a windy day so it made for some interesting shooting, but I'm very happy with it," said Arendz.
The 23-year-old originally thought he won silver for the second straight race, but he was elevated to the gold medal position on the podium after officials disqualified Russia's Vladislav Lekomtcev for having his coach on the range providing instructions.
"That is a big no no in our sport but I was surprised when they announced me as the winner," said Arendz.
Russia's Aleksandr Pronkov was second, followed by Norway's Nils-Erik Ulset.