Max Verstappen took the chequered flag, but at 20 years and 68 days, Antonelli beat Sebastian Vettel's previous record by more than three years. Vettel was 23 years and 134 days old when he won the first of his four titles with Red Bull in 2010.
He is also the first Italian world champion since Alberto Ascari in 1953, and Mercedes' first drivers' champion since Lewis Hamilton in 2020.
Antonelli arrived in Mexico 123 points clear of teammate George Russell and needing only a ninth-place finish to clinch the championship. He crossed the line in third place after Verstappen and Hamilton, respectively.
It is only Antonelli's second season in F1, but the youngster has been the sport's headline act since the spring: he won four races in a row from China to Canada, then won in Monaco, and took his first championship lead in Japan to become the youngest driver ever to top the standings.
In a race full of drama and incidents, including a full red flag and a safety car, the Italian kept his nose clean to finish on the podium behind former world champions Verstappen and Hamilton.
His teammate Russell finished in sixth, in what has turned out to be a miserable campaign for the British driver.
On the radio after the race, he was emotional speaking to his race engineer, Pete Bonnington, and celebrated on the track inside the stadium.
Speaking to Nico Rosberg, another world champion with Mercedes, after the race, he said: "We did it. Without my family and Mercedes, I wouldn't be here. This moment will live forever.
"It's been an incredible season."
Remarkable rise
The title triumph is the culmination of a remarkable rise for a man who headed into 2026 having never won an F1 race and hadn't driven an F1 car competitively prior to 2025.
Mercedes team boss Toto Wolff was always certain he had a generational talent on his hands, as he made clear when he chose Antonelli over a number of more experienced drivers when replacing seven-time world champion Hamilton for the 2025 campaign.
Many questioned the Austrian's judgement when his prodigy made a shaky start to his F1 career, making costly errors and crying in front of cameras after a poor qualifying performance in Belgium.
However, it was clear from the start of this year that the Italian had stepped up his game massively, and thanks to blistering pace, relentless consistency and remarkable composure in the face of immense pressure, he's put together one of the best campaigns a young talent has ever driven.
That's led to him becoming F1's youngest champion ever, Italy's third, and the nation's first since 1953.