Hirvonen in his Ford Focus won the rally in Värmland in western Sweden – the opener to the World Rally Championship season -finishing 42.3 seconds ahead of Loeb in a Citroen C4.
It was Hirvonen’s 99th race in the championship since he made his debut in 2002 and his 12th win.
The victory underlined a tremendous start to the season for the 29-year-old Hirvonen, who took the rival IRC Monte Carlo Rally in January.
“It’s absolutely fantastic!” he said. “And I’m really happy it’s over after those last few stages; you can’t imagine how nervous I was in there.
“It’s a perfect start to my year, I’ve never won the first round before, but after last season I’ve got one more year of experience.
“Now I plan to carry on like this. It’s a new game in Mexico and I can’t wait to get started again.”
Last year’s World Rally Championship runner-up took command of the rally from the fifth of the special stages opening up a lead of 10 seconds and he never looked back from there.
Loeb, who has won the last six world drivers titles, lost ground on the crucial fifth stage and his efforts to repair the damage were doomed after a poor choice of tyres saw him fall 17 seconds back at the start of the day.
After that he appeared to settle for a second-place finish with the rest of the season still to come.
“It’s been a tough weekend, and Mikko has done a perfect rally,” said Loeb.
“We made a little mistake yesterday, but even without it I don’t know if I would have been able to beat him in the stages. He was strong and he deserves his victory. I did my best. There’s nothing more to say.”
Third place went to another Finn in the shape of Jari-Matti Latvala in a Ford Focus, who won the Rally of Sweden last year.