The Grain Services Union traces its roots back to 1936 when elevator agents and office staff working for the Saskatchewan Wheat Pool formed the Saskatchewan Wheat Pool Employees' Association (SWPEA).
Over the years, SWPEA expanded to include all workers in Saskatchewan Wheat Pool's head office, elevator construction and repair division, terminal elevator offices, livestock division, and publications division.
In 1973, country elevator and construction employees of Manitoba Pool Elevators joined the union, and in 1974 the name was changed to the Grain Services Union.
Other groups of workers also organized to join the GSU: AgPro Grain terminal elevator employees in Saskatoon, Moose Jaw, and St. Boniface (formerly owned by Northern Sales and Elders Grain); AgPro Grain fish farm employees; Hillcrest Farms employees; Advanced Blueprint employees; and country elevator employees of Alberta Wheat Pool.
The Grain Services Union is affiliated to the Saskatchewan Federation of Labour, Alberta Federation of Labour, Manitoba Federation of Labour and was a direct affiliate to the Canadian Labour Congress until 1994. In that year, members voted to approve affiliating to the International Longshore and Warehouse Union (Canadian Area), although the connection to the CLC remains.