Andre Hainault and Kofi Sarkodie also scored in the second half after the Dynamo had spotted United an early 1-0 lead, keeping intact Houston's 26-game home unbeaten streak stretching back to last year.
The series now moves to Washington, where United hasn't lost since their season opener against Sporting Kansas City in March. But they will have to dig out of a two-goal hole to win the two-leg, aggregate-goal series.
Nick DeLeon got the only tally for D.C. United.
2014: The Year in Music [PHOTOS]