Lehtonen made 38 saves for Dallas. Los Angeles' Jonathan Quick had 23, but only two in the third period.
Dallas killed three penalties in the first 12 minutes.
"The penalty killing was fabulous in the first period," Ruff said. "I think that generated some energy for us, and led to us going the other way."
Later in that period, the Stars scored three goals in 5:23. Benn had a hand in all three.
They took a 1-0 lead at 14:35. On a 2-on-1 break, Seguin drew the defender to him and passed across the ice to Benn in the left circle. He had the left side of the net open for his second goal in three games after an 11-game drought.
Benn passed the puck to Klingberg at the top of the slot for his second goal in two games at 17:16. At 19:58, Klingberg passed back to Benn for a power-play goal from the left circle.
"We both had great opportunities in the first period," Kings coach Darryl Sutter said. "They scored on theirs and we didn't."
The lead grew to 4-0 when Lehtonen sent a pass up ice to Seguin at the Kings blue line for a breakaway goal-- his NHL-leading 15th tally.
The second of the Kings' three rebound goals, by Williams on the power play, pulled them within 4-2 at 12:16. He had the only goal in the third period, when Lehtonen shut out Los Angeles over the last 9:16.
"Once we decided to go there and shoot the puck, there was plenty there kicking around waiting for us," Williams said.
NOTES: Before the game, the Stars honored Mike Modano, their career scoring leader, for his induction into the Hockey Hall of Fame. Dallas also recognized Los Angeles assistant GM Rob Blake, who was also inducted. ... The teams finished the season series, with all three games played within 18 days. The Stars won the last two. ... The Kings entered with a 1.46 goals-against average at home, but 3.29 on the road. ... Stars C Jason Spezza played for the first time since signing a 4-year, $30-million contract extension Friday. ... Benn has four goals and 11 assists in his last 11 games against Los Angeles. ... Eakin has one goal and three assists in three games on a line with Benn and Seguin. ... Dallas has allowed a power-play goal in each of the last four games.