The union says talks with mill owner Resolute Forest Products and the Ontario government led the company to agree to keep the building heated.
The opposition parties earlier this month called on the government to change the rules on control of Crown forests to help save the mill, and up to one thousand jobs.
The Progressive Conservatives and NDP have said Resolute is blocking the sale of the shuttered mill to Expera Specialty Solutions of Wisconsin because it still controls the timber surrounding the town near the Manitoba border.
Natural Resources Minister Bill Mauro has said he understands the frustration of local residents who see timber being harvested and shipped elsewhere while their mill closes, even though there's a company that wants to buy it, and that "the system needs to change."
Unifor, which represents Resolute workers, in a release Saturday thanked the company for the winter heating and "their co-operation in ongoing efforts to find a buyer for the mill."
"Protecting this mill while we determine its future was an essential first step," Unifor national president Jerry Dias said.
Note to readers: This is a corrected story. An earlier version had a wrong name for Bill Mauro