In 1940, at a time when they had just come out of the depths of a great economic crisis, about fifty milk producers from St-Guillaume got together to create "La Société Coopérative Agricole de Beurrerie de St-Guillaume", now known as Agrilait. Together, they built the St-Guillaume Cheese Dairy: a butter and cheese manufacturing facility which was to become the engine of the cooperative.

The St-Guillaume Cheese Dairy processes approximately 22 million litres of milk annually into cheeses such as Cheddar, Brick, Monterey Jack, Swiss, and salted curled-style cheese curd.

The quality of the products from the St-Guillaume Cheese Dairy have allowed it to garner several prizes and distinctions, such as the "Grand Champion all Categories" for its flavoured Cheddar cheeses, the Canadian Western Premier Exhibition of Regina, as well as the honourable mention of "Reserve Champion all Categories" for its Swiss cheese, at the prestigious Royal Winter Fair in Toronto.