Lehtonen spent the 2004–05 NHL lockout season with the Wolves, adjusting to the North American playing style.

On the opening night of the 2005–06 season, Lehtonen suffered a serious groin injury when Florida Panthers centre Nathan Horton collided with him.

Shortly into the 2006–07 season, Lehtonen broke the Atlanta Thrashers shutout streak record previously held by Michael Garnett.

On February 8, 2007, Lehtonen became the Thrashers franchise leader for wins by a goaltender with 49, surpassing Pasi Nurminen's record of 48.

Prior to the 2015–16 season, the Stars acquired fellow Finnish goaltender Antti Niemi from the San Jose Sharks to compete with Lehtonen.

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 for a 4–3 defeat in the series.

[6] On July 2, 2011, Kari married longtime girlfriend Abbe Schutter in a private ceremony at the Club at Hammock Beach in Palm Coast, Florida.