In the following 2014–15 season, Niskanen won his first World Cup victory on 30 November 2014 by winning a 15 km classic race on home soil in Ruka, Finland.

On 1 March, he became the world champion on the 15 km classic event, winning the gold medal 17.9 seconds ahead of Norway's Martin Johnsrud Sundby.

In the WCH in Oberstdorf, he was in his season's best form, skiing a very strong 2nd leg in the relay and placing 6th in the 50 km classic, which he had won in the Winter Olympics three years earlier.

[7] In the 2021–22 season, Niskanen returned in stronger form and became the first Finnish male skier to reach the podium at the overall Tour de Ski, when he placed third.

By placing fourth in the 50 km race in Holmenkollen in the absence of his closest rivals Alexander Bolshunov and Johannes Høsflot Klæbo, he secured himself the Distance discipline globe already before the World Cup finals in Falun.

[8] He is also one of four Finnish athletes who have won gold in three different Olympics, summer or winter, after Paavo Nurmi, Veikko Hakulinen and Pertti Karppinen.

Iivo and Kerttu Niskanen are regarded as great sporting heroes in their childhood hometown of Vieremä, North Savo, which regularly hosts festivities following the siblings' Olympic and World Championship success.

At the season finale in Falun, Sweden, the siblings shared a World Cup podium for the first time when they skied the 1st and 3rd legs of the mixed relay event, where Finland placed second.