Auston Matthews, who snapped the second-longest goal drought of his career, Nikita Zaitsev and Mitch Marner, into an empty net, also scored for Toronto (30-17-2). William Nylander added three assists, while Morgan Rielly and Connor Brown had two each. Frederik Andersen stopped 41 shots for the win.
The Leafs came in having lost two straight, five of six at Scotiabank Arena — including four in a row — and seven of their last 10 overall.
Nicklas Backstrom, with a goal and an assist, Alex Ovechkin and Matt Niskanen replied for Washington (27-17-6), which was playing for the third time in four nights and lost its seventh consecutive game.
With his goal in the second period, Ovechkin tied Sergei Fedorov for top spot all-time among Russian-born players with 1,179 points.
Both the Leafs and Capitals don’t play again until Feb. 1 because of the NHL all-star break and their league-mandated bye week. Toronto defenceman Jake Gardiner missed a second straight game with back spasms.
Matthews, who had gone seven straight without a goal and had just one in his last 13, snapped a 2-2 tie with 3:41 left in the second when he ripped his 21st of the season through Holtby’s pads on a Toronto man advantage.
The goal was a huge relief for Matthews, who went 13 games without a goal as a rookie in 2016-17, and for the Leafs’ power play, which had gone just 6 for 52 since early December. John Tavares was somehow denied at the side of the net seconds before Matthews broke through.
Kadri made it 4-2 with his second of the evening at 2:10 of the third, blasting a one-timer off the rush beyond Holtby’s blocker.
The Toronto then centre completed the hat trick at 10:15. Nylander carried the puck into the Washington zone before dropping it to Brown, whose one-timer went off the crossbar and post before ricocheting to Kadri for him to deposit the puck into a wide-open net.
The defending Stanley Cup champions got one back with 2:46 to play when Niskanen’s shot went in off a Toronto player in front before Marner scored his 20th into an empty net with 32.6 seconds left.
Washington, which came in on an ugly 0-4-2 run, including Sunday’s 8-6 loss in Chicago and Tuesday’s 7-6 overtime defeat at home to San Jose, has now given up 21 goals in its last three games.
Kadri is also the third straight member of the opposition to record a hat trick against the Capitals after Chicago’s Jonathan Toews and San Jose’s Tomas Hertl.
Tied 1-1 after the first, the Capitals pushed ahead at 3:51 when Ovechkin blasted a shot from the top of the right face-off circle past Andersen’s blocker for his 37th goal of the season and record-tying point.
Washington’s captain leads all active NHLers with 38 goals against Toronto in his career despite sitting 11th in games played at 47.
Toronto woke up after that with a couple of spirited shifts before Zaitsev took a pass from Nylander in the slot and fired his first of the season and first in 53 games dating back to March 30, 2018.
Nylander, who has just one goal in 21 games since ending his contract impasse, played his best game of the campaign, accented by a nice backcheck on Ovechkin earlier in the shift before taking a hit and setting up Zaitsev.
Backstrom opened the scoring on a power play with 1:13 left in the first when he took advantage of some soft defending from Zaitsev in front to tap home his 13th.
Toronto got that one right back with 26 seconds remaining in the period when Rielly and Nylander pressured the puck behind the Washington net before it squirted in front. Holtby tried to cover the disc with his glove in the crease, but Kadri was quickest to the mark to poke his 11th over the line.
The Capitals got a scare earlier in the period when Ovechkin collided with Oshie near centre. Ovechkin stayed down for a few moments before leaving the bench as part of the NHL’s concussion protocol, but returned in time to be on the ice for Backstrom’s goal.
Notes: Fedorov played 1,248 regular-season games, while Wednesday was the 1,053rd appearance of Ovechkin’s career. … Capitals defenceman Brooks Orpik (rest) got the night off.