"I was coming to Canada to talk about prevention, to talk about my story. The goal is to dissuade young people from falling into a downward spiral," he said.
The Canadian airline, Air Transat, said its employees were obliged to apply the conditions of the US's Secure Flight programme when Benchellali tried to board the plane at the central French city of Lyon.
Although the Lyon to Montreal flight did not enter US airspace, according to flight tracking website FlightAware, the return flight would have flown over the US state of Maine.
The endowment fund Pace, which organised the peace conference Benchellali was to address, slammed the application of "inconsistent measures that are both unjust and ineffective and have been taken in an indiscriminate way in the fight against terrorism".
Benchellali, whose family is of Algerian origin, spent two years in the US's Guantanamo Bay detention centre after being captured in Afghanistan.
He was later jailed in France after a trial in which he admitted training with Al-Qaeda but denied fighting US forces or planning attacks.
After his release in 2007 he published a book about his experiences and lectures often in France, Switzerland and Belgium.
He often travels by plane in Europe but has never tried to cross the Atlantic before.