Luostarinen then cut it to 4-3 at 6:09 on a shorthanded breakaway, and Bennett tied it 4-4 at 12:01 when he deflected Aaron Ekblad's one-timer from the point on the power play.
"I was really happy for the power play," Panthers coach Andrew Brunette said. "I think they've really put a lot of pressure on themselves to make a difference. It was fun to see them get a couple, and obviously the shorthanded (goal) was huge."
Connor McMichael gave Washington a 1-0 lead at 5:52 of the first period when he banked the puck off the back of Bobrovsky's leg from behind the net.
Malenstyn scored 11 seconds later to make it 2-0, converting on a one-timer off a pass from Brett Leason.
Joe Thornton cut it to 2-1 at 6:55 with a tap-in into an open net after Samsonov stopped Ekblad's shot from the point. It was Thornton's 1,533rd career point, moving him into a tie with Mark Recchi for 12th in NHL history.
"For a majority of the game we played pretty well," Lomberg said. "They were just getting the bounces. It was good of our group to just stick with it."
Lars Eller extended Washington's lead to 3-1 at 6:40 of the second period when he skated around Ekblad and tucked a backhand inside the right post.
Nick Jensen made it 4-1 at 12:40 with a shot from the high slot that hit the stick of Panthers forward Anton Lundell.
"It shouldn't happen," Eller said of squandering the three-goal lead. "We even addressed it before going out for the third. Not good enough. ... We didn't execute. We kept playing a little slow and poor execution. We were behind, second to pucks, and we get penalties and they get the momentum."
NOTES:This was the fourth time the Panthers have rallied from three goals in the third period to win in their history. They also did it against the Columbus Blue Jackets on March 11, 2021, at the Boston Bruins on Nov. 12, 2019, and against the Toronto Maple Leafs on Feb. 10, 2009. … Malenstyn is the seventh Capitals rookie to score his first NHL goal this season, joining McMichael, Leason, Aliaksei Protas, Hendrix Lapierre, Martin Fehervary, and Garrett Pilon. … Thornton has 426 goals and 1,107 assists in 1,692 NHL games with the Boston Bruins, San Jose Sharks, Toronto Maple Leafs and Panthers.