Manchester United came to the meeting with an impressive 5-0 against RB Leipzig in the Champions League in the back.
But it was Arsenal who started a little more active at Old Trafford.
The away team had the two main chances in the first half that ended goalless.
Willian hit the top of the crossbar with a left shot while Bukayo Saka failed to get the ball on goal with his forehead, after just Wiliian found in with a nice post.
1-0 to Arsenal on penalty
In the second half, Arsenal continued to be the team that threatened the most, and it paid off with 20 minutes left.
Hector Bellerin collapsed in a duel with Paul Pogba and the referee pointed to the penalty spot.
From eleven meters, Pierre-Emerick Aubameyang was the safety himself and safely placed 1-0 at David de Gea's left post. The goal was the only one of the match and Arsenal thus took the first victory away against a top team since January 2015.