Reimer stopped three of the four shots he faced in the shootout after the Leafs were outshot 50-28 in regulation and overtime.
Toronto took the lead at 10:08 of the second period when David Clarkson scored on a power play. His redirection goal came on just the Leafs' second shot of the second period and 12th of the night.
Alex Ovechkin tied it with his NHL-leading 20th goal of the season at 15:50 of the third.
Eric Fehr scored in the shootout for the Capitals. James van Riemsdyk and Lupul scored for the Leafs.
Capitals center Mikhail Grabovski was playing his first game in Toronto since the Leafs bought out the final four years of his contract during the offseason. He didn't leave on great terms because of some disparaging comments he made about coach Randy Carlyle, but he expressed nothing but positive emotions Saturday about being back.
''I'm really happy to be here,'' Grabovski said. ''I'm happy to be around my friends who love me before and I love them, family, friends. Like home ice, my old home ice.''
Grabovski had a scary moment late in the second period when he took an inadvertent skate to the face from Clarkson as he was falling and the Leafs winger was skating by. Grabovski remained on the ice for a while as play continued and skated quickly to the bench after the whistle, leading to the crowd of 19,473 showering him with boos.
The Capitals said Grabovski needed 20 stitches to repair two separate cuts. He returned early in the third period.
Clarkson seemed to be everywhere Saturday night, making plays on a line with Nazem Kadri and Lupul.
It paid off on the power play in the middle frame.With Fehr in the penalty box for interference, Clarkson had a perfect tip of Jake Gardiner's shot from the point that beat Braden Holtby midway through the second.
The NHL's situation room took a look and called it a good goal.
''Video review was inconclusive in determining whether David Clarkson's stick contacted the puck below the height of the crossbar,'' the league said in a statement.
Just over a minute after scoring, Clarkson hit the outside of the post. Later in the period he took a hit from Capitals right winger Joel Ward and appeared to be in pain as he went to the bench, but he didn't miss a shift.
Clarkson also took an interference penalty early in the third.
It looked like Clarkson would be far more influential in the game than Ovechkin, who typically brings his A-game on ''Hockey Night in Canada.'' He was dangerously close to his season low in shots (three) but managed to get the job done.
NOTES: Toronto defenseman Paul Ranger was a healthy scratch as Mark Fraser returned after missing time with a knee injury. Ranger had played all 22 games going in and had one goal, five assists and a plus-4 rating. ... Martin Erat was scratched for Washington in favor of Fehr.