The 20-year-old striker kept up his good form and took over the MLS goalscoring lead with six on the season.
The Union were in front within seven minutes. Dejan Jakovic's careless pass was intercepted by Philadelphia, and Conor Casey slotted through to the in-form McInerney. The striker sliced through the D.C. defense before hammering home at the far post.
Just minutes later it got worse for the home side. Danny Cruz was sent down the right flank and cut past his marker before squaring for Conor Casey. The former Rapids striker cut onto his left boot before sending a low shot past Bill Hamid.
United would hit back quickly though. A cutting ball by Brandon McDonald found Dwayne De Rosario at the touchline. He squared back for Perry Kitchen to run onto the ball and unleash an unstoppable low drive from inside the box.
Incredibly, the goals kept on coming. United failed to deal with a long Union throw and McInerney again was on hand at the back post to control and finish clinically inside the box.
Right after the break, United got back in the game. Daniel Woolard was free on the left flank and he sent in an inviting cross which Lionard Pajoy nodded home from close range to make it 3-2.
That would be all from United though, and the club remains at the foot of the table with a 1-5-1 record.