The German was on hand to turn the ball home after home goalkeeper Julian Speroni could only palm Alexandre Lacazette's fierce effort straight into his path.
It was a simple finish in the end for the centre-back and it was a deserved goal for Arsenal, who had been dominating the game until that point.
The goal was Mustafi's second of the season, with the other having come against arch-rivals Tottenham back in November.
Check out the strike for yourself below:
GOAL ! Mustafi scores the rebound from Lacazettes shot. 1-0 Arsenal pic.twitter.com/xaYYCfQhrX
— ArsenalUpdates (@ArsenalEdits___) 28 December 2017