Lehkonen had 21 points (12 goals, nine assists) in 66 games with the Montreal Canadiens last season, missing time because of a back injury. He had 28 points (18 goals, 10 assists) in 73 games as a rookie in 2016-17, helping the Canadiens win the Atlantic Division with 103 points. 

In the Stanley Cup Playoffs, Lehkonen had four points (two goals, two assists) in the Canadiens loss to the New York Rangers in six games in the Eastern Conference First Round.

Lehkonen was selected by the Canadiens in the second round (No. 55) in the 2013 NHL Draft after playing for KalPa in the Finnish Liiga in 2012-2013. The Canadiens signed him to a three-year, entry-level contract on May 8, 2016. 

He made his NHL debut Oct. 13, 2016 in a 4-1 win against the Buffalo Sabres and scored his first NHL goal the following game, a 4-3 shootout loss to the Ottawa Senators on Oct. 15.

Before coming to the NHL, Lehkonen played two seasons for Frolunda in the Swedish Hockey League. In 2015-16, he had 33 points (16 goals, 17 assists) in 49 games and a team-record 19 points (11 goals, eight assists) in 16 playoff games to help Frolunda win the league championship. He also helped Frolunda win the 2015-16 Champions Hockey League tournament. 

Lehkonen had four points (two goals, two assists) in six games to help Finland win the gold medal at the 2014 IIHF World Junior Championship.