Wallace, who played for the Washington squad in 2009-10, broke a scoreless tie in the 21st minute when he stole the ball from defender Frederic Piquionne and fired home his fourth goal of the season.
Darlington Nagbe added a insurance tally in the 57th minute for the Timbers (5-1-7), who stretched their overall unbeaten streak to a club-record 11 straight games.
D.C. United (1-9-2), meanwhile, remained winless for a franchise-worst 11th straight contest.
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