The 20-year-old Italian achieved the historic milestone at the Autódromo Hermanos Rodríguez, finishing behind race winner Max Verstappen of Red Bull and Lewis Hamilton of Ferrari.
Antonelli entered the 71-lap race needing only a ninth-place finish to guarantee the championship, regardless of the results of his closest title rivals, Hamilton and Mercedes team-mate George Russell.
Starting fourth on the grid, the Mercedes youngster delivered a composed performance in a dramatic race that featured crashes, safety car interruptions and a red-flag stoppage.
He eventually crossed the finish line in third place, securing his maiden world championship at 20 years and 68 days old, breaking Sebastian Vettel's previous record of 23 years and 134 days.
The achievement also makes Antonelli the first Italian Formula 1 world champion since Alberto Ascari won the title in 1953, ending a 73-year wait for the country.
Following the race, an emotional Antonelli expressed his gratitude to his family and Mercedes team after achieving the biggest milestone of his young career.
"Thank you so much, we did it," he said over team radio during his slowing-down lap.
Speaking after the race, he added: "We did it. I am so grateful for this moment - and it is one I will live forever. I'm speechless, just trying to process what has happened."
He also described the campaign as an incredible season and expressed his happiness at sharing the podium with Verstappen and Hamilton.
Meanwhile, Verstappen claimed his sixth victory at the Mexico City circuit after holding off Hamilton in the closing stages to win by 0.799 seconds.
Hamilton finished second, while Antonelli completed the podium in third, 3.207 seconds behind the Dutchman.
Outgoing world champion Lando Norris finished fifth, while Antonelli's team-mate Russell came home sixth, ending his championship hopes.
The result officially confirmed Antonelli as the 2026 Formula 1 world champion, marking a remarkable achievement in only his second season in the sport.
Mexico City Grand Prix results
• 1st: Max Verstappen (Red Bull)
• 2nd: Lewis Hamilton (Ferrari)
• 3rd: Kimi Antonelli (Mercedes)
• 4th: Charles Leclerc (Ferrari)
• 5th: Lando Norris (McLaren)
• 6th: George Russell (Mercedes)
A historic Formula 1 achievement
At 20 years and 68 days, Antonelli becomes the youngest driver ever to win the Formula 1 Drivers' Championship, surpassing Sebastian Vettel's record set in 2010.
He also becomes Italy's first Formula 1 world champion since Alberto Ascari in 1953, adding his name to the history books after an outstanding 2026 campaign.