"I thought we did some really good things," Union manager John Hackworth said. "It was a day that we took care of business in front of goal and that was really great to see."
McInerney's first goal came just 7 minutes into the game, as he finished a textbook breakaway. Brian Carroll intercepted a clearance by D.C.'s Dejan Jakovic and fed Conor Casey, who split two United defenders with a through ball for McInerney. The 20-year-old caught up to the ball just as those two defenders were catching up to him, but he shook both off and ripped a shot past United goalkeeper Bill Hamid.
"Jack's capable of those kinds of those goals," Hackworth said. "I know people say that more than anything he's a tap-in specialist, but he has a lot of [skills] in his bag."
Four minutes later, the Union (3-2-2, 11 points) doubled the lead. Casey took a pass from Danny Cruz at the top of the 18-yard box and fired a low shot that Hamid could only get a fingertip on.
The sparse D.C. crowd of 12,349 - a shadow of the raucous gatherings that once were the envy of Major League Soccer - fell silent. All the noise came from the Sons of Ben, who traveled down I-95 and filled an entire section of RFK's north end-zone stands.
United (1-5-1, 4 points) got a goal back in the 17th minute, as young midfielder Perry Kitchen smashed home a setup pass from former MVP Dwayne De Rosario.
But just 9 minutes later, the Union had a two-goal lead again, as McInerney struck with a classic poacher's finish.
Sheanon Williams launched one of his trademark long throw-ins from the right side, Amobi Okugo flicked the ball along to McInerney in the 18-yard box, and he calmly shot home from the edge of a crowded 6-yard box.
That all of the Union's goals Sunday came from McInerney and Casey wasn't a coincidence. They form an ideal forward partnership: a big bruiser paired with a smaller, speedy dynamo.
"It's still early in the year, but when you're scoring goals it comes easy," Casey said of the duo's blossoming chemistry.
In the 47th minute, former Union forward Lionard Pajoy - a target of much criticism at PPL Park for his inability to finish chances - headed in a cross from Daniel Woolard to make the score 3-2.
D.C. pressed hard for an equalizer, bringing in fellow former Union forward Carlos Ruiz to play with Pajoy up top.
And then the Union started having trouble keeping possession of the ball. That has been a problem often this season, especially in midfield, and Hackworth has said so a few times. But again Hackworth was reluctant to bring in players who might help in that situation - such as Jose Kléberson or Roger Torres.
Hackworth acknowledged that bringing in Kléberson in the second half "was a thought" to help steady the midfield - and keep the ball away from D.C.'s surging attack. But Hackworth preferred the more defensive-oriented Michael Lahoud, and later Gabriel Farfan.
"We talked a lot about [Kléberson] on the bench, but it wasn't the right time to bring him into the game," Hackworth said.
There was one last chance for D.C. to tie the game when Union defender Ray Gaddis fouled United midfielder Chris Pontius just outside the 18-yard box. De Rosario stepped up to take the free kick, but he swung it right at MacMath, who caught the ball easily. A few seconds later, referee Hilario Grajeda blew his whistle to end the game.
There will be plenty of tougher challenges for the Union this year - after all, D.C. United is firmly rooted at the bottom of the East standings. But a three-goal performance is always welcome, no matter the opponent, and it showed the Union's potential to open up their offense.
Or, as Hackworth put it: "Our team is growing up."