2010-11: Artturi Lehkonen appeared in two games in Finland's U20 SM-Liiga as a 15-year-old, represented Finland in the 2011 U17 World Hockey Challenge and was one of the top scorers for TPS Turku's silver medal-winning U18 team. Playing for the TPS U20 team he had 1 assist and was minus-four in his brief stint. He scored 1 goal with 1 assist in five games for Team Finland at the WHC. Splitting the season between the TPS U18 and U20 teams, he scored 13 goals with 11 assists in 20 regular season U18 games. Lehkonen was second in points for TPS behind playmaking whiz Rasmus Kulmala in the playoffs and led the team with 8 goals in 13 games.
2011-12: Lehkonen had a breakout season, making his SM-Liiga debut with TPS, finishing as the fifth-leading scorer in Finland's U20 elite league and capping the year by leading all players with seven goals at the 2012 U18 World Junior Championship. Lehkonen scored 2 goals with 2 assists and was minus-seven with eight penalty minutes in 18 regular season games for TPS and was minus-one with no points in two playoff games. He scored 28 goals with 26 assists in 40 games and was plus-22 with 54 penalty minutes during the regular season for TPS' U20 team and scored 3 goals with 4 assists in four playoff games with TPS' U18 team. Lehkonen shared the points lead for Team Finland with Tappara's Henrik Haapala, scoring 7 goals with 3 assists and was plus-three with 6 penalty minutes in seven games at the U18 WJC. He was drafted by SKA St. Petersburg in the second round (57th overall) of the 2012 KHL Draft.
2012-13: Signed to a contract by KalPa following the 2011-12 season, Lehkonen was the Kuopio club's second-leading scorer and represented Finland in the 2013 U20 and U18 World Junior Championship tournaments. He scored 14 goals with 16 assists in 45 regular season games and was plus-12 with 12 penalty minutes as KalPa finished fifth in the 14-team league. Lehkonen scored 2 goals with 1 assist and was minus-two in four playoff games. In six games for Finland's U20 team he scored 3 goals with 1 assist and was plus-five with 4 penalty minutes. He was one of the key players for Finland's bronze medal-winning U18 squad, scoring 3 goals with 6 assists in seven games. Lehkonen was ranked ninth amongst European skaters in Central Scouting's final rankings prior to the 2013 NHL Draft.