Keystone Mushroom Farms, Inc. was a producer, canner, and distributer of mushrooms in Coatesville, Chester County, Pennsylvania. It was the successor of two previous businesses, L.F. Lambert Spawn Co. and Keystone Mushroom Company, Inc., which were formed in the early 1920s with complementary functions of spawn research; production and sales; and mushroom propagation, canning, and distribution.

The unifier was Louis Ferdinand Lambert (1866-1954), an emigrant Belgium mycologist who came to Chester County from St. Paul, Minnesota, in 1918, introducing the method of growing mushroom spawns from spores, rather than tissue, and looking for a market and a means of making this process profitable. After several unsuccessful business ventures, Lambert settled his spawn plant in Coatesville in 1922. His growing houses produced the first white mushrooms in 1925, a popular strain perfected by Lambert and trademarked "Snow White." His laboratories improved the process of growing and bottling mushroom spawn and sold it on a large scale locally, as well as to growers across the U.S. and abroad. L.F. Lambert Spawn Co. was maintained as a family-operated single proprietorship.

The Keystone Mushroom Company was incorporated in December 1920, at Pomerov (later Coatesville), Pennsylvania, to grow and can mushrooms on land purchased from L.F. Lambert. Lambert was a stockholder, and, while not among the first officers of the company, was elected treasurer in 1930, a position he held until his death. By 1936, Keystone was producing a line of canned mushroom products, including soup, broth, consommé, and spread, from the harvest of their houses and of area growers. These products were sold largely through commission brokers from stock warehoused throughout the U.S. In 1941, Keystone leased the growing houses of the Lambert Spawn Co., and by this annually renewable agreement, the two functioned as a unit under the supervision of the management of the Keystone Mushroom Company.

During World War II, Mr. Lambert's work in mycology led to a War Production Board contract to produce penicillin in the company's laboratories. When Lambert died in 1954, his companies were consolidated under the name Keystone Mushroom Farms, Inc. In 1956, the business was incorporated.