In the greatest final of all time, Argentina won 4-2 on penalties after a 3-3 draw against France.
Messi opened the scoring from the spot to give Lionel Scallone’s side the lead in added time, before slotting home in the penalty shootout.
Argentina would have won had Messi’s Paris Saint-Germain team-mate Kylian Mbappe not scored a hat-trick for the reigning champions.
But it was the 35-year-old who eventually captained La Albiceleste to victory, lifting the trophy on a Qatar night that also saw him pick up the Ballon d’Or for player of the tournament.
Asked what made the difference in the final, Brighton midfielder McAllister said: “It’s always Messi, always Messi. He’s fantastic.
“Words can’t explain how I feel right now, I think this team gave everything on the field, so I’m very happy with the result.”
Mac Allister himself impressed in the final and played a big part in setting up Angel Di Maria’s goal as Argentina went two goals up at half-time.
They pulled back Mbappe’s 97-second brace to send the game into extra-time and replaced McAllister moments before the French forward completed a hat-trick to cancel out Messi’s extra-time opener.
With Messi already adding football’s biggest prize to his already packed trophy cabinet, he has, in McAllister’s eyes, sealed his place as the best.
“I think he’s the best player in the world, the best player of all time,” he told ITV Sport.
He’s amazing, he’s a really good guy, he’s humble, so he’s got everything to be the best player in history, so it’s amazing to share the dressing room with him and I’m very grateful.
McAllister’s sentiment was echoed by teammate Rodrigo de Paul, the Atletico Madrid man, who told Gol Mundial: “Lionel Messi is the best in history. There’s no doubt about that.
“He showed it the whole game, he showed it the whole tournament. He was the best ever.
“He showed it even at 35. The best captain I’ve ever seen in my life.”