Lehner first represented Sweden internationally at the 2009 IIHF World U18 Championships in Minnesota. He was the starting goaltender in the tournament, helping Sweden to a fifth-place title while recording a .916 SV% and a 2.80 GAA. Lehner recorded one shutout in the tournament, blocking 20 shots in a 7–0 opening-round victory over the Czech Republic. Lehner next played for the Swedish junior team at the 2011 World Junior Ice Hockey Championships in Western New York. He got the start in Sweden's semifinal game against Russia, which Sweden lost 4–3 in a shootout. Lehner criticized the referees after the game, saying that he had missed Russia's second goal because he had expected an icing call on the play.