McInerney found himself all alone at the far post and when the cross was coming he told teammate Amobi Okugo to let it go.
"Amobi was going up with, I think, [Daniel] Woolard and I screamed 'Leave it' - and half the time they don't do it but, thank goodness, he listened."
With so much at stake against a team with the worst MLS record, the Union were flat for much of the first half and portions of the second.
After saying he was proud of how the Union fought back, team manager John Hackworth didn't give the overall performance a passing grade.
"When we perform well I'm OK, but that wasn't a great performance by the Union tonight," Hackworth said.
The Union are 12-10-10 with 46 points and two games remaining. Montreal and Chicago also have 46 points. Technically Montreal is in fourth place, Chicago in fifth, and the Union in sixth due to tiebreakers. The top five teams in the Eastern Conference will make the playoffs.
Montreal, which has three games remaining, hosts the Union on Saturday. Chicago has two remaining games.
Union midfielder Brian Carroll was playing his 300th regular season MLS game. Carroll, 32, began his MLS career with D.C. United.
"It's cool and I like the fact I've been able to accomplish it - but a win would have been nicer," Carroll said.
D.C. United (3-22-7) didn't play like a team with the worst MLS record
After dominating much of the first half, United finally took a 1-0 lead when Nick DeLeon hit a blast from 30 yards out into the upper left corner in the 36th minute.
DeLeon had plenty of space after taking a pass from Lewis Neal.
DeLeon was able to take two dribbles and then fire on net, with no Union defender near the goal-scorer.
That began a flurry of activity.
In the 37th minute United keeper Bill Hamid made a sensational save off a Conor Casey header. Casey got the rebound and shot from six yards out but Hamid made a diving save at the near post.
Then in the 38th minute, United's Conor Doyle hit the right post with a shot from outside the box.
As the second half began, United continued to control play. United almost put the game away, but Chris Pontius, with Union keeper Zac MacMath out of the goal, missed an open net in the 83d minute.
The Union were playing without midfielder Fabinho (red card) and defender Sheanon Williams (yellow car accumulation).
Midfielder-forward Sebastien Le Toux started in the back. Le Toux, who has been bothered by plantar fasciitis, could no longer push off the foot according to Hackworth and was replaced in the 16th minute by Matt Kassel.