The 2,700 workers are members of Unifor, the new union created by the merger of the Canadian Auto Workers with the Communications, Energy and Paperworkers union.
Unifor announced the agreement in a release Saturday morning that said the tentative deal was negotiated late Friday.
Details of the agreement are being withheld pending the results of a ratification vote on Sunday.
The results of the vote are expected to be released Sunday evening.
The CAMI workers are represented by Unifor Local 88, and manufacture the Chevrolet Equinox and GMC Terrain vehicles.
Local bargaining committee chair Mike Van Boekel said he expects the proposed four-year deal will be approved by union members.
"I am extremely confident that it will pass," he said.
An early round of contract talks began back in February in the hopes a deal could be reached by March 24, but the union said that negotiations needed more time. The current contract was set to expire Monday.
In March, GM announced it was investing $250 million in new equipment at the plant so it could build a wider variety of vehicles at a lower cost.
North American automakers have been gaining momentum recently as the industry recovers from the severe downturn that started in late 2008.
Automakers posted their best sales in a decade last year, up almost six per cent, and the CAMI plant's two vehicles were strong sellers in GM's lineup.
Industry observers have predicted 2013 North American sales will surpass last year's as the U.S. economy revives from the recession.