Niskanen first appeared at the 2010 Junior World Championships in Hinterzarten . Over 10 km classic he reached seventh place. In the 2010/11 season he took part in the Scandinavian Cup , but where he only achieved placements in midfield. Nevertheless, at the end of the season he got his World Cup debut in Lahti . He had to end the 20 km skiathlon race prematurely because he was lapped. One day later, he finished 66th in the sprint. At the 2011 Junior World Championships in Otepää , he won bronze with the relay.
In the 2011/12 season he took part in some races of the Scandinavian Cup and junior races. At the Junior World Championships 2012 in Erzurum , he was fourth over 10 km classic. Niskanen took part in the Finnish Junior Championship and won two golds and one silver. At the beginning of the 2012/13 season , 40th place over 15 km was his best result. Participation in the Scandinavian Cup and national championships followed. He won gold twice at the Finnish Junior Championship in 2013.
In the 2013/14 season , Niskanen first competed in the Scandinavian Cup. In January 2014 he won silver in the sprint at the Finnish Cross Country Ski Championships. At the end of January he won gold over 15 km classic at the U23 World Championships in Val di Fiemme . He was then allowed to take part in the World Cup again at the beginning of February and achieved eighth place in Dobbiaco over 15 km and thus World Cup points for the first time. Because of these good results he was nominated for the Finnish Olympic team. At the 2014 Olympic Games in Sochi , he finished fourth over 15 km classic and missed a medal by 0.2 seconds. In the team sprint he won gold together with Sami Jauhojärvi . At the start of the 2014/15 season in Ruka , he clinched his first classic World Cup victory over 15 km. Placements in midfield followed. He ended the Tour de Ski 2015 prematurely. At the 2015 World Championships in Falun , he came in 26th place in the skiathlon and eighth place in the relay. In the U23 classification he reached fourth place. At the beginning of the 2015/16 season he came 27th at the Nordic Opening in Ruka . His best placement of the season in the World Cup was 12th in the 50 km mass start race in Oslo . At the beginning of April 2016, he was at the Finnish championships in Taivalvaara over 7.5 km classic and second in the pursuit race. A week later he became Finnish champion over 50 km in Ruka.
At the first World Cup of the 2016/17 season in Ruka , Niskanen clinched his second World Cup victory over 15 km. This was followed by tenth place in the World Cup mini tour in Lillehammer . In January 2017 he won his first classic Scandinavian Cup race in Lahti over 15 km. The following month he was second in the 15 km classic at the World Cup in Otepää . At the season highlight, the 2017 World Championships in Lahti, he won the bronze medal together with Sami Jauhojärvi in the team sprint and the gold medal over 15 km classic. He also achieved fifth place with the relay. At the end of the season, he finished second in the 50 km mass start race in Oslo and came 14th in the overall World Cup and fourth in the Distance World Cup. At the start of the 2017/18 season, he finished sixth in the Ruka Triple . He was third over 15 km classic. This was followed by seven top ten placements, including third place in Lahti over 15 km classic and thus reached 15th place in the overall World Cup and 13th place in the Distance World Cup. At the 2018 Winter Olympics in Pyeongchang , he won the gold medal over 50 km in classic style. In April 2018 he became Finnish champion over 50 km classic in Taivalvaara .
After finishing seventh in the Lillehammer Triple at the beginning of the 2018/19 season, Niskanen won the Scandinavian Cup in Vuokatti and the World Cup in Otepää, each over 15 km classic. In addition, he achieved third place in Lahti together with Ristomatti Hakola in the team sprint and in Cogne the second place over 15 km classic and thus reached 20th place in the overall World Cup and 13th place in the Distance World Cup. At the season highlight, the 2019 World Championships in Seefeld in Tirol , he won the bronze medal as defending champion over 15 kilometers in the classic technique behind the Norwegian Martin Johnsrud Sundby and the Russian Alexander Bessmertnych , after just missing a medal four days earlier in the skiathlon in fourth would have. At the Finnish championships in Äänekoski and Ylitornio he triumphed over 15 km classic, in the pursuit race over 15 km freestyle and over 50 km freestyle. At the beginning of the 2019/20 season, he took third place in the Ruka Triple. He won the classic 15 km. He finished the Tour de Ski 2019/20 with three top ten finishes, including third place in the pursuit in Toblach , in tenth place. This was followed by second place over 15 km freestyle in Nové Město and in the pursuit in Trondheim on the 2020 ski tour, which he finished in eighth place. At the end of the season he won the classic 15 km in Lahti and finally achieved sixth place in the overall World Cup and third place in the Distance World Cup.