Reimer turned aside 18 shots in the third period but yielded Alex Ovechkin's 20th goal of the season with 4:10 remaining, which tied the score 1-1.
A final try by the Capitals' Troy Brouwer missed the mark and Toronto emerged with a third win in four games.
David Clarkson got the Leafs' goal in regulation.
Holtby stopped 27-of-28 shots as Washington dropped a third straight.
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