International Cycling Union President Pat McQuaid said it was "very annoying and very frustrating" when riders made excuses after being caught.
"It's obviously better and takes more character if a rider can come out and say that he made a mistake," McQuaid told The Associated Press in an interview. "Then accept that mistake and accept his punishment and come back."
McQuaid suggested that accused riders follow Millar's example after he accepted a punishment and campaigned for clean cycling.
Millar completed a two-year ban in 2006, after confessing that he used the blood-booster EPO, then joined the World Anti-Doping Agency's athlete committee.
On Saturday, Millar will ride for a five-man Britain team in the road race trying to bring sprint specialist Mark Cavendish home with the first gold medal of the London Games.
The Scottish racer's Olympics debut offers unexpected redemption at age 35.
Millar got his chance after two landmark legal rulings by the Court of Arbitration for Sport overturned Olympic rules which sought to exclude athletes with doping convictions.
His value to the British team was shown when he served as road captain for a squad including Tour winner Bradley Wiggins which carried Cavendish to a world championship victory last year in Copenhagen, Denmark.
Millar proved his current form when winning the longest stage of the Tour de France this month.
"I'm an ex-doper and I'm clean now, and I want to show everyone that it's possible to win clean on the Tour," Millar said in Annonay, France.
After several recent Tours were blighted by doping scandals, just one case has so far been recorded this year. Frank Schleck of Luxembourg tested positive for a little-known diuretic called Xipamide.
"We still have athletes that make stupid mistakes," McQuaid said. "You know, (Frank) Schleck on the Tour this year and we'll have to wait and see what the story is and the eventual outcome is."
The UCI president referred to the circumstances of Floyd Landis being stripped of the 2006 Tour title, though without naming the disgraced American cyclist.
Another American rider, Tyler Hamilton, also admitted doping during his career after years of denials. The International Olympic Committee will decide in the coming weeks whether to strip Hamilton of his time-trial gold medal from the 2004 Athens Games.
"They did it a lot in the past, make all sorts of excuses and to lie, lie and continue to lie in court and in front of tribunals — and then to come two years later, and admit that they lied for two years," McQuaid said. "It gets very annoying when a cyclist gets caught for a recognized doping substance and then he tries to, you know, make all sorts of excuses."
Landis and Hamilton have both alleged doping by their former teammate Lance Armstrong, the seven-time Tour winner. A case is now being pursued by the U.S Anti-Doping Agency.
"In relation to the Armstrong affair — which is hanging over our heads even though it goes back 15 years or so — it's a negative hanging over our heads and the sooner that is settled and sorted, we can move forward," McQuaid said.