Fielding a second-string unit ahead of the U.S. Open Cup final, United started the match brightly but was picked apart by Toronto after halftime.
Jared Jeffrey opened the scoring for United in spectacular fashion, but strikes from Darel Russell, Bright Dike and Alvaro Rey, as well as a D.C. own goal, gave Toronto the victory.
With the U.S. Open Cup final at Real Salt Lake looming Tuesday, United coach Ben Olsen fielded a second-string team and only dressed five substitutes. Among the starters for United were 17-year-old Michael Seaton and 18-year-old Collin Martin, homegrown players signed this year.
United took the lead 15 minutes in. Given time and space 30 yards out, Jeffrey set his sights on goal and curled a gorgeous strike into the upper corner.
Toronto drew even 11 minutes later. Collecting a deflected service from Bobby Convey, Russell chested the ball down and hit a no-look volley past Joe Willis.
The home side jumped ahead in the 67th minute. With the referee playing advantage following a Marcelo Saragosa foul, Toronto right back Mark Bloom whipped in a cross that Dike volleyed home.
Toronto added an insurance goal four minutes later, as a centering feed from Rey deflected off United's Conor Shanosky for an own goal. And Rey wrapped the scoring in the 87th minute when he capped an individual run with a far-post finish.