D.C. (10-5-4) has won two straight since snapping a two-game skid. The United padded their lead in the Eastern Conference to 10 points over New England. The Fire (4-9-2) have dropped four straight, their longest losing streak of the season.
After United's Bobby Boswell kicked the ball right into a defender, the rebound rolled back to Doyle, who fired a strike from outside the 18-yard box and past diving goalkeeper Sean Johnson. The shot hit under the crossbar and deflected in for Doyle's second goal of the season.
Boswell attempted a header from just outside the 6-yard box early in the second half, but was denied when Johnson made a diving stop and deflected it away.
Washington hasn't won at Chicago since 2009.