The man's attorney says the plot was a "jihadi fantasy" and that his client knew nothing about it.
A judge ruled Thursday that evidence of a terrorist group was "overwhelming." The man is the first person to be found guilty of a terrorist offense in Canada since the country enacted anti-terrorism laws in 2001.
The arrests of the 18 group members, known as the "Toronto 18," made headlines around the world and heightened fears in Canada, where people believe they are relatively immune from terrorist strikes.
Prosecutors said there were plans to truck-bomb nuclear power plants and a building housing Canada's spy service.
Seven of those arrested have since had their charges either withdrawn, or stayed. The trials of 10 adults, including the alleged ringleaders, have yet to begin. The young man was the first to go on trial.
Superior Court Justice John Sproat found the man guilty of knowingly participating in a terrorist group. As the 94-page judgment was handed down, the defendant's mother wept quietly in the back of the court.
The man has not been identified because he was 17, a legal minor, when he was arrested in 2006. He is now 20.
Prosecutors argued he attended a training camp where he participated in military exercises and firearms training and that he knowingly participated in a potentially deadly conspiracy. He had pleaded not guilty to terrorism-related charges.
Sproat rejected the defense argument that the plot was a "jihadi fantasy" that the defendant knew nothing about.
"He clearly understood the camp was for terrorist purposes," he said.
The defense, with the help of the prosecution's star witness, had cast the plot as "musings and fantasies" with no possibility of being carried out.
Sproat however said few would have believed the Sept. 11 attacks in the U.S. were a possibility before they happened.
"I'm satisfied beyond a reasonable doubt that a terrorist group existed," Sproat said.
Sproat rejected defense arguments that two camps organized by the alleged ringleaders were simply a religious retreat or recreational in nature. "Apparently benign activities may be used to identify and indoctrinate recruits," he said.
Sproat noted that participants marched, played paintball games, shot a handgun and heard lectures on waging war against the West. He also called the young man an "acolyte" of the "charismatic" ringleader.
Evidence was clear the youth listened carefully to his mentor, the plot's ringleader, and wanted to please him, and therefore understood what the camps were about, the judge said.
"He had a full appreciation of the nature of the terrorist group," Sproat said.
Defense lawyer Mitchell Chernovsky said it's hard to know what sentence will be imposed but said his client was involved peripherally and doesn't have a criminal record.
The prosecution's star witness, Mubin Shaikh, infiltrated and spied on the alleged terror cell members before their arrests. Shaikh is a former Canadian army cadet. Chernovsky had noted that even Shaikh testified that his client was unaware of a plot.
Shaikh said outside court that the youth should not have been found guilty. Shaikh called the man a "naive Muslim kid who fell into the wrong circle of Muslim kids."
"I don't believe he's a terrorist. I don't believe he should have been put through what he was put through, but that's our system," Shaikh said.
Shaikh, however, was happy the judge found his testimony about the alleged ringleaders credible. Shaikh received about $300,000 for infiltrating the group.
Sproat noted that the defense did not make any suggestion that the payments influenced Shaikh's evidence. Sproat said he found Shaikh to be a truthful and reliable witness, a development that doesn't bode well for the adults in their trials.
"I've been telling the truth since day 1," Shaikh said. "I'm very happy that the judge validated that and confirmed that. That will carry through to the remaining adult trials.