Letang gave the Penguins a 2-1 lead at 15:23 of the third period. Guentzel skated in on a 3-on-1 before sending a cross-ice pass to Bryan Rust, who quickly passed back to Letang for the goal. Letang missed the previous 11 games because of an upper-body injury.
The Penguins killed two penalties in overtime, and Jack Johnson made a save with his skate on Sebastian Aho's shot at an open net in the final minute.
"We were just in desperation mode there in the penalty kill," Johnson said. "We were doing whatever we can to keep the puck out of the net."
Mrazek, who is 7-1-0 in his past eight starts, made a number of key saves throughout the game, including two on Nick Bjugstad in the first period, and on Garrett Wilson, Joseph Blandisi and Matt Cullen in a flurry around the net in the third.
"He's phenomenal," Williams said. "He's a gamer and I love his enthusiasm in there. As a player on the bench, when he makes a big save and he gets jacked up like that, it gets you going."