"I do not believe the testimony of the accused," Court of Queen's Bench Justice Christopher Mainella said in an oral decision that lasted for two hours.
"The accused was not credible about his behaviour. He ... provided nonsensical answers and was not consistent in his version of events."
Storheim, who became the top Canadian cleric for the Orthodox Church in America, was accused of sexually assaulting two brothers in the summer of 1985, when he was a priest in Winnipeg. The brothers, whose identities are protected by a publication ban, lived with Storheim for brief periods, on separate occasions, while they worked as altar boys.
They testified Storheim would walk around naked and asked to be touched sexually. One brother told court Storheim got into bed with him while naked. That brother, however, admitted to memory problems and a history of mental illness and alcohol addiction. Mainella found Storheim not guilty on that charge.
The other brother was very detailed in his recollection of events and was deemed "a credible witness" by the judge. The brother, now in his 30s, said Storheim once combed through the boy's pelvic area to look for pubic hair, examined his pyjamas for semen stains and had the boy touch Storheim's genitals.
Storheim denied any inappropriate touching when he testified. He told court he was telling the boy about puberty based on some Biblical teachings.
The judge rejected Storheim's version of events.
"The accused would have no justification to touch (the boy) as he did," Mainella said.
"This goes beyond a mere misguided discussion about puberty."
Storheim, who is now 68, sat passively but was attentive as the judge delivered the verdict. A small group of supporters hugged Storheim afterward.
A date has not yet been set for sentencing. Outside court, defence lawyer Jeff Gindin said he is likely to ask that Storheim be spared any jail time and to seek probation or a conditional sentence.
"He has no record, and on the scale of sexual assaults, this would be certainly on a lower level compared to some that you see."
Gindin said he may also appeal the conviction, but will wait for the sentence before making a decision.
Storheim was arrested in 2010 after the brothers went to police with their complaint. He was placed on leave.
The Orthodox Church in America has 700 parishes, missions and other institutions across North America. It is separate from other churches such as the Greek Orthodox Church and the Ukrainian Orthodox Church.