Rookie Drew Doughty also scored for the Kings, who outshot Washington 32-21 and snapped Alex Ovechkin's five-game goal streak. Michal Handzus and Anze Kopitar scored empty-net goals in the final 40 seconds.
Nicklas Backstrom and Brooks Laich scored for the Capitals, who had won six of their previous seven games and hadn't taken fewer than 27 shots on net in any of their previous 18 games contests this season.
Washington played without defenseman Mike Green, sidelined by a shoulder injury after being checked hard into the end boards by Anaheim's Chris Pronger during the first period of Wednesday's 6-4 win. Alexander Semin, the Capitals' leading scorer, missed his third consecutive game because of an undisclosed upper-body injury and Sergei Fedorov missed his seventh straight with a sprained ankle.
The Capitals, who came in averaging 31.6 shots on net and had scored at least one goal in 13 consecutive regulation periods, were outshot 12-3 in the first period by a Kings squad that entered the game allowing a league-low 24.1 shots per game. Alex Ovechkin had all three of Washington's shots against Eric Ersberg, who made his eighth straight start and has allowed 16 goals during that stretch.
The only goal in the opening period was by Doughty, the second overall pick in the draft, whose low slap slot from just inside the blue line deflected off Ovechkin's stick and over Brent Johnson's glove at 13:58.
The Capitals received a two-man advantage at 3:54 of the second period when Raitis Ivanans was sent off for holding Milan Jurcina, and Oscar Moller was penalized for hooking Ovechkin after the delayed penalty against Ivanans. Backstrom tied the score 54 seconds later at the right of the crease, converting Viktor Kozlov's perfect pass through the legs of defenseman Denis Gauthier.
The Kings came up empty during a 5-on-3 power play they had for 1:23 later in the second, but they took a 2-1 lead at 18:25 of the period when O'Sullivan got a lead pass from Matt Greene and beat Johnson between the pads with a short wrist shot from the right circle after a turnover by Ovechkin back in the Los Angeles zone.
Ovechkin was serving an interference penalty when Kings defenseman Kyle Quincey carried the puck the length of the ice and dished off at the last instant to Brown at the edge of the crease for his seventh goal and a 3-1 lead.
Saturday's game at San Jose will mark the one-year anniversary of Bruce Boudreau's hiring as Washington's coach. The Capitals had the NHL's worst record when he took over from Glen Hanlon on Thanksgiving Day and were off to their worst start in 26 years, but they won 15 of their final 19 games and secured a playoff berth on the final day of the season with a 3-1 victory over Florida - earning Boudreau Coach of the Year honors.
Notes: With Ovechkin in town, the Kings proclaimed it "Russian Heritage Night" at Staples Center. On hand to drop a ceremonial first puck between Ovechkin and Alexander Frolov was newly inducted Hall of Famer Igor Larionov. Frolov and Ovechkin both were born in Moscow. ... Backstrom, who turns 21 on Sunday, has scored a goal in four of his last five games after a season-opening 14-game drought. His 69 points last season were second among rookies. ... The Capitals faced the Kings on the road for the first time since Dec. 14, 2005. The only players still on their roster from that game are Ovechkin, Laich, Matt Bradley, Chris Clark and Shaone Morrisonn. The only players still on the Kings' roster are Frolov, Brown, Derek Armstrong and goalie Jason LaBarbera. ... The only teams other than Los Angeles that Ovechkin hasn't scored a goal against are San Jose and Minnesota - the Capitals' next two opponents. ... Ovechkin had an assist on Backstrom's goal, his 15th point in six games. Ovechkin scored 65 goals last season to break Luc Robitaille's NHL record for left wings.