Lehner didn't strap on goalie pads until age 10, but by the time he was 15 he was stopping pucks for the Swedish under-16 team, as well as for Frolunda in the Swedish Elite League's under-18 division.

He won a league championship with Frolunda's under-20 team in the 2007-08 season, and in 2008-09 played for Sweden at the Under-18 World Championship. The Ottawa Senators selected Lehner in the second round (No. 46) of the 2009 NHL Draft.

Lehner then headed to North America and played for Sault Ste. Marie of the Ontario Hockey League in 2009-10. He also made his professional debut that season, starting two games for Binghamton of the American Hockey League and winning both.

Lehner made his NHL debut Oct. 16, 2010, playing the opening 4:42 of the third period of a 4-3 loss to the Montreal Canadiens, stopping all three shots he faced. When he got the start Jan. 13, 2011, against the New York Islanders, Lehner became the youngest goalie in Senators history at 19 years, 173 days. He stopped 20 shots in a 6-4 win, his first NHL victory.

He continued to split time between Ottawa and Binghamton through 2012-13. In 2010-11 he helped Binghamton win the Calder Cup, going 14-1 with a 2.10 goals-against average and a .939 save percentage in the postseason and winning the Jack A. Butterfield Trophy as the AHL playoff MVP.

On Feb. 28, 2012, Lehner made 32 saves for his first NHL shutout, a 1-0 win against the Boston Bruins. On Feb. 4, 2014, he became the first goaltender in Senators history to face 50 shots in one game and earn a victory, making 46 saves on 50 shots in a 5-4 shootout win against the St. Louis Blues.

On June 26, 2015, Ottawa traded Lehner and David Legwand to the Buffalo Sabres for the No. 21 pick in the 2015 NHL Draft.