The 23-year old forward opened the scoring in the 28th minute when he chased down a deflected ball to the byline before cutting back and slotting past Tally Hall at the near post.
The Dynamo recovered courtesy of Bonieck Garcia's 44th minute effort before Adu converted from the penalty spot five minutes into the second half prior to Josue Martinez wrapping things up 20 minutes left.
Adu was thrilled to have made a contribution to the Union's well overdue victory but he was slightly disappointed to come off with a minor quad strain which he sustained whilst taking the penalty.
"Trust me, I really wanted to be out there," said Adu who admits it has been a frustrating season. "But sometimes you've got to take care of yourself, too. I'm just glad we won the game.
"It happens a lot to me (coming off early) - I do feel like that, yeah," he added. "It is very frustrating. But this is due to different circumstances.
"Health is your No. 1 priority. You want to live to play other games and you want to be able to play them at 100 percent. I could have made it worse, I really could have. I had to make a very tough decision in coming out."