A moment of madness from Paul Pogba allowed Arsenal to win from the penalty spot for their first Old Trafford victory in 14 years.
Arsenal star Pierre-Emerick Aubameyang kept his composure as the Gunners won away at a top six club for the first time since 2015.
Aubameyang came close to opening the scoring early on when he narrowly failed to get on the end of a cross from Willian.
The Brazilian then turned from provider to serious goal threat as he rattled the crossbar with a stinging shot from range.
Arsenal continued to dominate and next up it was Saka who headed over from close range.
Lacazette and Aubameyang combined well at the start of the second half with Aubameyang firing narrowly wide.
But then it was a clumsy bit of defending from Paul Pogba on Hector Bellerin and Aubameyang had no trouble sending David de Gea the wrong way and converting his penalty.
The goal ended a five game Premier League goal drought for the Arsenal striker.