McGrath, who looks set to end the season on a high after the low of crashing out of the Olympic slalom with gold in his grasp, extended his lead over Norwegian-born Brazilian Lucas Pinheiro Braathen to 41 points with one home race remaining.
Former teammate Pinheiro Braathen finished third in Kranjska Gora with Norway's Henrik Kristoffersen second, 0.01 slower than McGrath.
The final slalom of the season is in Hafjell on March 25th.
"So many times in my career I've been on the wrong side of the hundredths," said McGrath. "I've lost five victories with a combined two-tenths before.
"There's no better place I could fight for the Globe. Me and Lucas, we grew up skiing together in Hafjell. So it has a lot of special memories, and it's going to be a pretty cool fight until the end."
McGrath was fastest in the first run, with Braathen 0.17 slower, and Belgian Armand Marchant third and eyeing only a second career podium.
Marchant then skied out of the second run while Kristoffersen, fourth after the first run, set the pace and stayed ahead of Braathen by 0.03. Austrian Michael Matt was fourth, 0.06 off the lead.
The top six were separated by only 0.12 after the two runs.
"I was so nervous before the second run," said McGrath. "I thought way too much about the overall standing, but hundredths were on my side, and I'm so happy with it."
The Norwegian dedicated the win to his late grandfather, who died on the day of the Milano Cortina Olympic opening ceremony last month.
"I miss him a lot, and to bounce back like this after the Olympics, I think he has something to do with it," he said. "He was looking over me today."