In his decision, Ontario Superior Court Justice John McMahon accepted that Kelly Pflug-Back, of Guelph, Ont., at one point gave instructions to vandals dressed in black as they moved up Yonge Street smashing windows.
McMahon's ruling came after the Crown argued Pflug-Back had a leadership role that should be considered as "an aggravating feature" in her sentencing.
Pflug-Back, who is in her early 20s, pleaded guilty in February to charges of wearing a disguise and seven counts of mischief over $5,000.
Final sentencing submissions will be heard July 13, when the Crown is expected to seek a sentence of 18 to 24 months, while the defence wants a conditional sentence.
At Monday's pre-sentencing hearing, the Crown tried to prove Pflug-Back was one of the "directing minds" of the vandalism that erupted downtown.
"Ms. Pflug-Back was more than just a sheep," prosecutor Liz Nadeau told court. "She was definitely one of the leaders."
The Crown's lone witness, Det. Const. Andrew Hassall testified that Pflug-Back yelled "Hurry up!" as several of the so-called Black Bloc members smashed a Bell store window.
About four or five vandals, who were at the front end of a large group of protesters, then stopped outside a nearby perfume store.
"Leave it alone. We only want big business," Hassall quoted her as yelling.
"They spared it after Ms. Pflug-Back said what she said."
The group then attacked a Tim Hortons store, he testified.
Pflug-Back, who could be seen in video and some of the 6,000 photographs of the protest that Hassall took — several of which were screened for McMahon — was dressed in black and had her face covered.
Among the photographs, Pflug-Back, distinguishable by a silver belt with three skulls as a buckle, can be seen hitting a police vehicle window and an ATM screen with a metre-long wooden stick.
Court also saw videos of some of the destruction in which Pflug-Back could be seen taking part.
Under cross-examination by defence lawyer Steve Gehl, Hassall was adamant he heard Pflug-Back yelling at the Bell and perfume stores, and said the group in front of the window looked to her for direction.
"I believe they are verbatim comments," he said.
He admitted he only wrote down her words several months later, when asked to provide a complete statement to police, something Gehl told McMahon made his testimony suspect.
The judge said he believed the officer's account.
After the violence, dozens of protesters, including Pflug-Back, slipped out of their black clothing and donned "civilian" clothing, the officer told court.
"Ms. Pflug was the centre of attention during the time people were changing," Hassall said.
The judge discounted that evidence.
During the lunch break, some shoving occurred outside court as supporters of Pflug-Back used umbrellas to shield her from news cameras. She retreated into the courthouse.
The case is one of the few victories for police, who have faced biting criticism for their handling of the tumultuous summit in June 2010. More than 1,100 people were arrested, most to be released without charge.
Outside court, Det. Sgt. Gary Giroux, who helped lead a task force aimed at bringing the G20 vandals to justice, said he was pleased with McMahon's ruling.
"She took an active leadership role in directing individuals to do damage," Giroux said.
About 40 people have been successfully prosecuted for the vandalism, that included burned police cars and smashed windows. Canada is also asking the U.S. to hand over five Americans to face G20 charges.