Granit Xhaka beat David de Gea with a swerving shot from outside the penalty area, which the United keeper was expected to stop. United improved towards the last minutes of the first half but Arsenal held on and after the break, it was Arsenal who added the second goal through Pierre-Emerick Aubameyang who scored from the spot after Alexander Lacazette was fouled by Fred.
The Gunners have won seven out of 10 top-flight games in 2019 and are one point behind north London rivals Tottenham, who sit third in the table.
For a manager in his first Premier League season, undertaking a considerable rebuilding exercise on limited resources, Unai Emery is doing a very good job. It was a strange kind of victory — the first goal an unfamiliar sight, the second all too common.
Nobody expects a howler from David de Gea these days, a contentious penalty decision, sadly, seems to happen every game. Arsenal benefitted from both.
That is not to say the win was without merit. Sead Kolasinac was the man of the match, a constant threat from left wing-back, closely followed by goalkeeper Bernd Leno, who was both quick-witted and brave in repelling United's forwards.
Ainsley Maitland-Niles is a prospect, while Granit Xhaka is at last looking a central midfielder capable of holding the fort as well as blasting his way out of it.
Paul Pogba has been running games of late but not this one. It wasn't that he didn't play well, more that Arsenal kept him in check. There remains the potential for upset, as happened in Rennes last Thursday, but gradually Emery is getting his team where he wants it.