On September 4, 2012, the day before an impending lockout, Lehtonen signed a five-year, $29.5 million contract. Lehtonen made the playoffs with the Stars for the first time in 2014. Prior to the 2015–16 season, the Stars acquired fellow Finnish goaltender Antti Niemi to compete with Lehtonen. They alternated regularly the next two seasons. In the 2016 playoffs, Lehtonen established himself as the primary goalie. However, in Game 7 of the second round, he was pulled after giving up three goals in the first period as the Stars' season ended with a 6–1 loss to the St. Louis Blues.