The former Albion midfielder opened the scoring with a header as Argentina beat Switzerland 3-1 in the quarter-finals.
They will face England in the semis in Atlanta on Wednesday.
Mac Allister was starring for the Seagulls when he helped Argentina win the 2022 World Cup in Qatar.
Albion fans and colleagues were cheering him on throughout the tournament and he was given a hero’s reception when he returned to the Lancing training ground.
Mac Allister is now among the players who stand in their way after a Jude Bellingham double saw England past Norway.
The Liverpool midfielder said: “From the first day I arrived in England – it has been six years now, if I remember correctly - people have always treated me incredibly well.
“I have nothing against that.
“We understand all the history in the background, which is perhaps not the fault of the fans, but of those in positions of power, but we understand that it's going to be a very special match.
“From my side, I always have respect for them. But, obviously, on the pitch, we will give our best."
Mac Allister headed home a Lionel Messi corner on ten minutes for Argentina’s fastest goal of the tournament so far.
Dan Ndoye levelled on 77 minutes but Switzerland then had Breel Embolo sent off.
Julian Alvarez and Lautaro Martinez scored for Argentina in the second period of extra-time.