Tuesday's semi-final had been cagey, with Croatia having most of the possession, before Alvarez burst through to chase a long ball and was brought down by goalkeeper Dominik Livakovic for a penalty that Messi converted in the 34th minute.
Five minutes later, Alvarez again charged from the halfway line, rode his luck as two attempted tackles merely kept the ball bobbling in his path, and nudged in the second goal.
He put the game beyond reach in the 70th minute with a simple finish after Messi had produced a piece of magical control to hit the byline and pull the ball back for his delighted 22-year-old team mate.
While Messi will now hope to bow out as an absolute superstar of the world game with the one trophy to elude him on his 172nd appearance in Sunday's final, Alvarez is at the other end of his career.
IN HAALAND'S SHADOW
After signing for Manchester City on his 22nd birthday he has hardly set the Premier League alight, perhaps unfortunate to be striving to make his name in the shadow of Erling Haaland, chalking up three goals in 12 less-than-memorable appearances.
On the international front, he arrived in Qatar with three goals to his name and behind Lautaro Martinez in the pecking order to play alongside Messi.
Argentina's stunning defeat by Saudi Arabia in the opener shook things up though and when he got his first start of the tournament in the third group game against Poland, he grabbed the opportunity by scoring in the 2-0 win.
He added another in the 2-1 win over Australia and on Tuesday became the youngest player to score two in a World Cup semi-final or final match since 17-year-old Pele in 1958, taking his international tally to seven and virtually securing him a starting berth in Sunday's final.
"Today I think Julian was a standout above all else. He played an extraordinary game," Messi said.
"He opened the way for us, he fought, he ran, he hit everyone. I think he was extremely important as he has been throughout the World Cup."
Despite scoring two goals and earning the penalty for the other, Alvarez was bizarrely overlooked for the man of the match award, with FIFA seemingly contractually obliged to give it to Messi whenever he takes the field.
He will not care a jot about that, however, as he enjoys the ride of his life.
"I'm happy personally and for the group," he said. "We have to rest, we hope to have a great game on Sunday. "Calchin (his home town), like the whole country, must be going crazy. We are happy for what we are achieving and we are going for more."
