Mainoo scores twice for United U21s in dramatic 3-3 draw
Mainoo put United 1-0 up in the first half before the visitors equalised on the hour mark to make it 1-1 at the interval.
The Seagulls went ahead in the second half but Mainoo notched his second of the game to make it 2-2 in the 54th minute.
United dropped behind again on 67 minutes but battled back again once more when Bjorn Hardley earned a share of the spoils.
Mainoo nets twice against Brighton
The starting XI for the game was: Vitek; Jurado, Bennett, Fredricson, Hardley; Collyer, Savage; Shoretire, Mainoo, Hansen; McNeill
On the substitutes bench for the Under 21s, who beat Barrow in the EFL Trophy in midweek, were: Mastny, Murray, Huddlestone, Mather.
Goalkeeper Radek Vitek made an impressive double save in the first half and that proved crucial in a game which ebbed and flowed.
It was 17-year-old Mainoo who shone though, although his efforts were not enough to earn United all three points on the day.
The result means that United are still waiting for their first league win of the season at this level, drawing five times and losing thrice.
The youngsters showed good character to twice battle back from behind in their first league game since losing 6-1 to their Blackburn counterparts last weekend.
Next up are Chelsea next weekend, as United continue their search for that first victory and United need to improve a defensive record which has seen them concede the most goals in the division with 24.