Justice Guylene Beauge ruled Friday the Mounties must provide the information to the Charbonneau Commission.
The RCMP had been trying to get out of parting with information gathered during Operation Colisee, a wide-ranging investigation targeting the Montreal Mafia.
The force argued it didn't need to co-operate with a provincial commission and called the inquiry's request "unmanageable."
The inquiry's lawyers countered that the information was necessary for the probe, which will explore allegations of corruption involving the construction industry, organized crime, political parties and the awarding of public contracts.
In a 14-page ruling, Beauge rejected the force's arguments that the federal agency could not be compelled to co-operate with a provincial inquiry that the subpoena was too broad, and that there were concerns about confidentiality of sources and third parties.
In ordering the Mounties to appear before France Charbonneau and provide the information requested, Beauge wrote that the commission has the right to call on the RCMP — a public body that is accountable.
"There is no privilege or restriction that prevents it from handing over information," Beauge wrote.
She called the RCMP involvement "a fundamental civic obligation."
Citing a Supreme Court of Canada decision, Beauge wrote that commissions are "often convened, in the wake of public shock, horror, disillusionment, or skepticism, in order to uncover 'the truth.'"
"They are an excellent means of informing and educating concerned members of the public," she said, quoting from a previous judgment.
The RCMP also argued the subpoena it was served with was too vague given the massive amount of material it had gathered — 1.5 million wiretap conversations, 1,500 hours of video, along with still pictures.
But the commission said its blanket subpoena was only a formality and insisted it was looking for much more specific information, in particular on 17 individuals and seven companies.
Commission lawyers went to court after failing to get the information voluntarily, in late 2011 and again earlier this year.
Beauge ruled the subpoena is precise enough. Any further requests can be more specific and debated, if need be, on a case-by-case basis.
She was also assured the inquiry would be able to protect sources as needed.
The information the inquiry is seeking was gathered during Operation Colisee, a police probe into Montreal's Italian Mafia that concluded in 2006 and is considered the largest investigation into the Mob in Canadian history.
Charbonneau, a Quebec Superior Court colleague of Beauge's, is to begin hearing from witnesses this fall.