Frenchman Noel led after the first run but lost time with a scrappy start to his second and could not better the time of McGrath, who won with a combined time of 1:44.50. Noel was 0.30 seconds back with Swiss Loic Meillard in third, 0.39 seconds slower.
It was the 18th time that Noel had led after the first leg of a World Cup slalom, but he has converted those into victories only seven times, and it proved another day of frustration for the 28-year-old, who was only 22nd-quickest on his second descent.
US-born McGrath produced a solid second run and accelerated through the closing gates to claim his fourth World Cup victory.
"I am so incredibly happy. This feels absolutely insane,” said McGrath, who will be one of the favourites at the 2026 Olympics.
Norwegian-born Brazilian Lucas Pinheiro Braathen was third-quickest in the second run at the Italian resort but just missed out on a podium place in fifth position, with Norway's Timon Haugen, the slalom World Cup leader, in fourth.
