Jesse Marsch's side found themselves with a mountain to climb inside half an hour when Luke Ayling was shown a straight red after a quickfire brace by Eddie Nketiah gave the hosts an early, comfortable lead.
With ten men, United put on an admirable showing to chase a result after the interval but Diego Llorente's second-half goal proved only a consolation as the Whites failed to find an equaliser in North London.
The defeat allowed Everton to climb out of the relegation zone with a 2-1 victory at Leicester City, causing the Whites to drop below the dreaded dotted line, level on points with 17th-placed Burnley but with a significantly poorer goal difference than their Lancashire counterparts.
Graham Smyth, the YEP's chief football writer, hands out the scores from Sunday afternoon's clash at the Emirates.
1. Illan Meslier
4 - Culpable for the first with a dreadful error. Looked shaky for a few set-pieces and distribution even before the red wasn't great.
2. Luke Ayling
2 - Struggled with Martinelli then got himself sent off with an abomination of a tackle in the first half an hour. A dreadful, costly performance.
3. Robin Koch
5 - Battled but seemed affected by the desperate state of the first half performance. Ended up as a right winger.
4. Diego Llorente
4 - Popped up with a goal but had some really shaky moments as Arsenal pressed in the first and second halves.
5. Junior Firpo
3 - Given a horrible time of it by Saka. Miraculously escaped without a booking. Provided the flick on for the goal.
6. Kalvin Phillips
5 - Chased shadows for much of it but dug in and fought. Couldn't influence the game on the ball.