It was the leak of an explosive report by Duchesneau in the fall of 2011 that finally pushed Charest to name a provincial inquiry into corruption after resisting public pressure for almost two years.
Now, with the inquiry on recess until after the Sept. 4 election, the probe's star witness may be holding forth from the hustings as a candidate for the Coalition for Quebec's Future.
Corruption, one of the key issues in the election, has been a longtime preoccupation for Duchesneau, who apparently did some of his investigating for the provincial anti-collusion squad on his own time.
"It's time we asked if we have played Russian Roulette with our ethics in recent times," he wrote in a text addressed to business ethics students at the Universite du Quebec a Chicoutimi, which is posted on his personal website. "It seems to me that we have. The proof of our passivity is obvious. . . .
"The moment of truth has arrived. We now need clear, strong measures and a general mobilization to put the brakes on collusion."
It was a corruption investigation that put Duchesneau into the public eye in the 1980s. Then a young sergeant-detective, he arrested his own boss on the Montreal police drug squad for stealing cocaine and hashish from an evidence locker.
The supervisor, a legend touted as a future chief, went to jail.
Earlier this year, some commentators called on Duchesneau to make a second try to become mayor of Montreal but he apparently wasn't interested. He was defeated in his previous mayoral bid, his only attempt at elected politics.
Not everybody has always been that keen on Duchesneau — who can be blunt, perhaps even abrasive.
Lawyers at Quebec's corruption inquiry gave him an unprecedented grilling on the minute details of his government contract, even asking if there was a restroom in his office. This was after explosive testimony in which he made allegations of widespread corruption but didn't name names.
"The enemy is the people I spent 18 months tracking," he replied in frustration at hearings in June.
"All these questions are really funny. We point out collusion to you and what you're looking at is my finger, not where we should be going."
Duchesneau, 63, reportedly rubbed his boss the wrong way at the anti-collusion unit when he maintained the province's anti-corruption squad should be headed by a judge. The man in charge happens to be a former Quebec provincial police officer.
Montreal Mayor Gerald Tremblay also demanded an apology from Duchesneau after he made disparaging comments about Tremblay in a newspaper interview this year. He joked that he wouldn't trust the mayor to sell him a used bicycle.
In Duchesneau's bid for the mayor's job in 1998, he described opponent Jean Dore as arrogant and condescending, and as the mayor who had plunged Montreal into fiscal hell.
He was kinder to the then-incumbent, saying Pierre Bourque was a nice man but his Vision Montreal team "is falling apart and he is not the man for the job."
But a group called Les Entartistes (the pie throwers) expressed their own opinion of Duchesneau by planting a cream pie on his face at his mayoralty campaign kickoff.
"I like cream but not in the face," he quipped at the time.
Duchesneau's political views during the mayor's race sound similar to those promoted by the CAQ now. When a newspaper editorial called him a strong federalist, he was quick to announce he was neither federalist or separatist, just pro-Montreal.
In the past, Duchesneau has donated to Charest's Liberal party. Earlier this year, he gave $1,000 to the new party he's running for, the Coalition.
Duchesneau is no stranger to corruption allegations himself.
He was accused in media reports of breaking Quebec election laws when he ran unsuccessfully for mayor.
He stepped down briefly from leading the anti-collusion squad to fight the allegations, vehemently denying he had concealed an illegal loan to his campaign.
Duchesneau, who had been dubbed "Mr. Clean," was cleared of any wrongdoing and rejoined the anti-collusion until he was fired. He has since accused certain media members of having a personal agenda against him.
The product of a modest Montreal family, Duchesneau started out as a street cop in 1968. He worked his way up through the ranks to the chief's job in 1994.
He took over at a time when the force was wracked by union unrest, some officers were under fire for beating a taxi driver into a coma and relations with the black community were at a low ebb.
He described the job then as "like being the captain of a ship sailing into a storm."
His five years in the post were marked by bringing professionalism back to the troubled force, instituting community policing and cracking down on organized crime, particularly outlaw bikers.
He would occasionally patrol in a squad car and helped out on the street during Quebec's devastating ice storm that blacked out much of Montreal in 1998.
After his failed bid for mayor, the Canadian Army reserve officer worked as a consultant and businessman before being tapped in 2002 to head the Canadian Air Transport Security Authority following the Sept. 11, 2001, terrorist attacks in the United States.
He stayed there until 2008 when he left to complete his doctorate in counter-terrorism and was named by Charest to the anti-collusion task force in 2010.