Brown shot the Calgary mother three times in her northeast home. The Crown alleges he did it on behalf of Belanger's estranged common-law husband.
The couple had a son, and Belanger was seeking sole custody of the boy.
The defence argued Belanger was the victim of a mystery killer and is raising doubts about DNA evidence presented during the trial.
Brown was arrested during a traffic stop in 2010, and charged with second-degree murder. The charge was later upgraded to first-degree murder.
First-degree murder carries an automatic life sentence. Victim impact statements and sentencing arguments will be heard Monday.
Lily Belanger, the victim's mother, spoke outside of the Calgary courtroom after the verdict.
"This is wonderful and it has been a long time — 12 years, 12 years next month," she said.
Brown showed little reaction as the verdict was read out, but some members of his family burst into tears.
"He doesn't feel anything," said Lily Belanger. "He's got no emotion at all, so he's nothing."
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