Second-half goals by Jurrien Timber and William Saliba condemned new head coach Ruben Amorim to a first defeat, on a wet, inhospitable night in the capital.
The Gunners' threat from corners is well documented and they looked capable of netting from most of them, even without injured chief aerial presence Gabriel.
It made for an uncomfortable challenge that the Reds were unfortunately unable to pass.
FIRST HALF - DALOT GOES CLOSEST
United did well to quieten the expectant home crowd, particularly after having the ball in our net very early on. Andre Onana's misplaced pass was punished by Gabriel Martinelli but Kai Havertz was clearly offside and it was rightly chalked off.