The omitted national team striker finally found the net for his new team, providing a vital goal.
Jack McInerney put the Quebecois side up in the 56th minute. Finishing up a lightning-quick attacking move, the striker turned in Justin Mapp's endline cross after the winger did well to get to the ball.
Johnson scored the equalizer in the 84th minute. Fabian Espindola launched a cross in from the left wing and Johnson's jump was largely uncontested as he thumped a header home for his first goal of the year and first in a D.C. United shirt.
United is now 4-3-2 this season, while last-place Montreal is 1-5-3.