Preston Crown Court heard that Sarah Williams, 35, shot Sadie Hartley, who was 60, with a 500,000-volt stun gun as she answered the door at her home in Helmshore, Lancashire, Sky News reports.
Williams worked at Crystal Ski Holidays at Manchester’s Chill Factor indoor ski centre.
Her friend, Katrina Walsh, 56, was also convicted of murder for helping to plot the brutal attack on January 14.
Williams stabbed Hartley, inflicting more than 50 injuries, Sky News reports.
Williams had become obsessed with Hartley’s partner, 57-year-old former fireman Ian Johnston, with whom she had had a brief fling.
Johnston had ended the relationship but the pair had continued exchanging text messages.
Williams was arrested three days after the murder. Blood was found in her bath and in the getaway car.
Detective Superintendent Paul Withers, who led the investigation for Lancashire Police, said: "This murder was nothing short of the cold-blooded, premeditated and carefully planned assassination of an entirely innocent woman and I welcome today’s verdicts.
"While it may have been Sarah Williams who carried out the actual killing of Sadie Hartley, there can be no doubt that Katrina Walsh helped her every step of the way and was up to her neck in the planning of this brutal slaying. They are both as culpable as each other."
Crystal Ski Holidays said it did not have a comment to give.
