Receiving the ball from Aaron Wan-Bissaka on the half-turn, our no.37 took one touch before smashing a delicious drive that sailed into the top corner to beat Caoimhin Kelleher.
Peeling away to celebrate in front of the Stretford End, it was another moment savour for the youngster, as his first senior strike at Old Trafford.
The game eventually finished 2-2 but most will be remember the occasion given the teenager's superb shot.
Netting his third goal of the season in the process, Mainoo has continued to go from strength to strength under Erik ten Hag's guidance in the first team this season.