Lino Zambito's pleas came as his trial was slated to begin in Saint-Jerome today.
He pleaded guilty to six charges including fraud, conspiracy and intimidation stemming from contracts awarded in Boisbriand, north of Montreal.
Zambito will have to wait until Sept. 29 for sentencing arguments as the trial for a co-accused began today.
He is best known for his 2012 testimony at Quebec's Charbonneau Commission where he described a cartel-like structure that drove up the cost of projects in Montreal and Laval.
Zambito told the inquiry that businessmen were paying kickbacks to municipal parties and paying a cut to the Mafia.
Defence lawyer Conrad Lord says he expects discussions with the Crown about sentencing to take place in the coming months.
Zambito himself took to Twitter on Wednesday today to say he wouldn't comment on the pleas out of respect for the judicial process.