But Dallas lost three players to injuries.
Lehtonen made the Stars' two first-period goals hold up for their third straight win. The Panthers have lost on consecutive nights.
Jamie Benn scored Dallas' third short-handed goal in two games; the Stars lead the NHL with nine short-handed goals.
Jason Spezza scored for the 2-0 lead.
Tyler Seguin, Dallas' leader with 29 goals and 59 points, was helped off the ice after being checked in the lower leg by defenseman Dmitry Kulikov in the third period. Patrick Eaves followed Seguin to the locker room 51 seconds later after teammate John Klingberg's wrist shot hit Eaves in the head.
A third Stars forward, Ales Hemsky, left in the first period with a lower body injury.
Florida goalie Al Montoya had 26 saves.