Friday 13th was a lucky day for the veteran Scot, who will anchor a strong British team at the Olympics in London later this month and gave his home country their fourth stage victory in this edition of the Tour.
"We've become one of the top nations. We have the yellow jersey, we have the world champion and me, the old dog, winning the stage here. We're at the top. Our Olympic team is pretty damn scary," said Millar, a former doping offender turned anti-doping campaigner.
Another key member of the team will be Bradley Wiggins, who retained his overall lead and spent the whole day in the front of the bunch, showing he had become the rightful boss of the peloton.
Millar's win, at 35, means all the more with the Games so close as four of the five members of the British Olympic team for London have now won a stage in this Tour.
"It's also especially important for me as it comes exactly 45 years after Tommy Simpson's death. I'm an ex-doper and I'm showing everyone you can come back clean and win a stage," added the Garmin-Sharp rider, whose last of three previous stage wins on the Tour dated back to 2003.
"I want people to have trust in our sport. Cycling has changed a great deal. I want people to understand that," he said.
Former world champion Simpson died when he collapsed on Mount Ventoux in 1967 after taking amphetamines.
Millar was given a two-year suspension in 2004 after confessing to drug-taking and he nearly missed his eleventh Tour de France when a stomach bug confined him to bed on the eve of the prologue in Liege a fortnight ago.
The Scot, whose team have been depleted by crashes since the start, improved with each day however, and joined the successful break on the two first-category climbs in the early stages of the long, 266-km ride from St Jean de Maurienne.
In the tough Granier pass, 80 kms into the stage, only five men were left at the front - Millar, Spaniard Egoi Martinez, Croatian Robert Kiserlovski and Frenchmen Jean-Christophe Peraud and Cyril Gautier.
None of them being threats overall, they were left free to stay ahead and go for stage victory.
Their lead steadily increased and was still seven minutes 53 seconds when they reached the line for the final battle.
Peraud, a mountain bike silver-medallist at the Beijing Olympics, attacked with two kilometres to go and was immediately reined in by Millar, who easily outsprinted him for stage victory. Martinez was third.
At the back, it was almost business as usual for Team Sky, with the slight difference that Wiggins himself took the reins when it mattered.
The yellow jersey holder moved away near the top of the Granier, catching his rivals off guard, and later said he had just gone to chase a potential overall contender.
"I did it just to help the team and take the pressure off the guys, who had been working hard for quite a while," he said.
Overall, the Briton retained his 2:05 lead over team mate Chris Froome, another stage winner in this Tour.
After the last big climb in the Alps, Frenchman David Moncoutie, a four-time King of the mountains in the Spanish Vuelta, crashed out of the race on the descent.
World champion Mark Cavendish, overshadowed by his higher-priority Sky team mates since his victory in the first stage, could face a heavy fine after clashing with a race steward as he tried to make it back into the peloton at the back of a team car after a puncture.
Saturday's 13th stage takes the peloton towards the Mediterranean coast after a 217-km ride between St Paul Trois Chateaux and Le Cap d'Agde.
