Michael Briscoe has been found guilty of first-degree murder for killing 13-year-old Nina Courtepatte in April 2005.
However, Justice Keith Yamauchi said the Crown failed to prove Briscoe was involved in the murder of prostitute Ellie May Meyer.
Briscoe, 41, who was on trial for both deaths, was also convicted of kidnapping and sexual assault in the Courtepatte slaying. He was given an automatic life sentence with no chance of parole for 25 years.
In his 108-page decision, Yamauchi said Briscoe was not a credible witness, going as far as to say Briscoe lied several times during his testimony.
Courtepatte's mother, Peacha Atkinson, cried out as Yamauchi announced his decision. Ellie May Meyer's parents sat nearby in the front row of the crowded courtroom.
Atkinson said outside court that the conviction of the last person involved in her daughter's death is important.
"It's still bittersweet. It still doesn't bring Nina back," she said. "I still can't get to hold her except through my pictures but it's a good ending."
Courtepatte's battered body was found on a golf course west of Edmonton in April 2005, while Meyer was found in a farmer's field east of the city in May.
Police believe Meyer was killed two days before Courtepatte.
Briscoe denied participating in the brutal sexual attack and murder of Courtepatte, although he admitted driving the teen, along with a group of young people, to a golf course west of the city.
Yamauchi said while Briscoe did not kill or sexually assault Courtepatte, he knew what his accomplice, Joseph Laboucan, planned to do.
Laboucan was convicted in both the Meyer and Courtepatte slayings.
While Briscoe's ex-girlfriend Stephanie Bird testified during the trial she saw Briscoe and Laboucan beating Meyer, Yamauchi did not believe the Crown proved beyond a reasonable doubt Briscoe had struck her.
Briscoe was acquitted in Courtepatte's death after a trial in 2007, but the decision was later overturned by the Supreme Court of Canada and a new trial was ordered.