The concept of developing a large, world-class zoological park in Chicago’s western suburbs dates back to the Colombian Exposition of 1893. It was not until 1919, when philanthropist Edith Rockefeller McCormick (daughter of John D. Rockefeller) made her donation of 80 acres to the Forest Preserves of Cook County that planning began in earnest. The Forest Preserves donated the remaining land, and the Chicago Zoological Society, a private not-for-profit entity, was chartered in 1921, entering into a public-private agreement with the Forest Preserves to develop, operate and maintain the Chicago Zoological Park (subsequently adopting “Brookfield Zoo” as its informal name) as a private non-profit entity.

When it opened in 1934, the Zoo was the first-of-its-kind modern "bar-less" zoological park in North America, in which animals were cared for in naturalistic settings by using moats to separate animals from guests. Today, the partnership between Brookfield Zoo Chicago and the Forest Preserve District of Cook County is considered one of the most successful relationships of its kind in the nation.

FPDCC website