The England youth international striker suffered a knee injury against Middlesbrough back in August and was back in the starting XI after a couple of substitute appearances.
Mason (pictured right) was on the end of Kyle-Fraser Allen's cross in the 10th minute, Watford levelled 10 minutes later before Mason struck again this time after good work from Fraser-Allen and Andros Townsend.
After host of chances came and went, David Hutton finally added the third goal on 65 minutes when he converted from the penalty spot after being upended himself in the box. Watford bagged a late second.
Spurs U18s: Button, Mtandari (Clare, 80), Berchiche, Parrett, C Butcher, Caulker (Kasim, 70), Fraser-Allen, Hutchins, Obika, Mason (Hutton, 60), Townsend. Unused subs: Dalton, Olsen.
Ryan Mason marked his first start since the opening day of the campaign with two goals as our Under-18s beat Watford 3-2 on Saturday.