The 17-year-old winger lashed home from 25 yards with only six minutes remaining, cancelling out Fabio Zamblera's 60th minute opener for the hosts.
In an even first half at a blustery training ground, it was Danny Drinkwater who had United's best efforts. First, his long range effort was caught by Magpies goalkeeper Ole Soderberg, before the young stopper was forced to parry out the midfielder's left foot volley from the edge of the area.
Newcastle always looked threatening through the pace of Kazenga LuaLua, but it was through a set-piece that they took the lead on the hour mark.
Again Drinkwater was involved, as he inadvertantly flicked on Patrick McLaughlin's right-wing corner and Italian striker Zamblera was on hand to tap in from close range.
United responded in gritty fashion, however, and after Chris Curran had seen his shot deflected over and Matt James' deflected shot had rattled the crossbar, the Reds salvaged a point through Stewart.
Seizing on defensive hesitancy, the Manchester-born winger advanced on goal and sent an unstoppable right-footed shot crashing into the top corner of Soderberg's goal.
United: Amos; Derbyshire, Gill, C Evans, James; Norwood (Brown 58), Curran, Drinkwater, Galbraith (Dudgeon 58); Brandao (Stewart 46), Macheda