[4] The zoo is noted for the second birth of polar bears[5] and siamangs[6] in captivity and for their locally famous gorilla Samson, who lived from 1950 to 1981 and whose bones are now on display at the Milwaukee Public Museum.

The Milwaukee County Zoo opened in 1958 with the primate building, monkey island, feline house, pachyderm mall, and grizzly bear den.

[29] In 1986, the children's zoo was renamed the Stackner Heritage Farm, and a dairy complex was constructed as part of it, to celebrate Wisconsin as America's Dairyland.

This fifteen-thousand gallon tank features a group of Humboldt penguins with underwater viewing glass surrounding it.

As of 2018, the zoo will have a total of four Western lowland gorillas, after the death of 31-year-old dominant male Cassius and 17-year-old Naku in April.

It was remodeled in 2002 to include a large viewing deck for zoo guests and an expanded shelter for the resident macaques, with waterfalls and a larger mountain.

[37] This building holds invertebrates, fish, reptiles, and amphibians[44] including Chinese alligators, green anacondas, Gila monsters, red-tailed boas, panther chameleons, several poison dart frogs, tomato frogs, king cobras, several other species of reptiles, amphibians, freshwater and saltwater fish.

[46] This building, located near the Aquatic and Reptile Center, features a special room for nocturnal species that is darkened in the day and brightened at night so the animals live on a schedule friendly to zoo visitors.

Residents include fennec fox, prehensile-tailed porcupine, Prevost's squirrel, Pygmy slow loris, common vampire bat, ring-tailed lemur, straw-coloured fruit bat, Mohol bushbaby, Southern three-banded armadillo, Goeldi's monkeys, cotton-top tamarins, golden lion tamarins and a larger exhibit for Hoffmann's two-toed sloths.

[46] Other species are sometimes rotated in and out, such as the zoo's red pandas during construction of the Florence Mila Borchert Big Cat Country building in 2004.

[46] Many of these species like the reindeer and the grizzlys are displayed in predator-prey "panorama" exhibits, which make them appear to share the space via a hidden moat.

The newly renovated children's zoo with a focus on native midwestern United States wildlife and educational presentations, featuring species such as hedgehogs, domestic ducks, North American porcupine, red-tailed hawk and barred owl.

A series out of outdoor exhibits featuring animals from South America, Asia, and Africa, along with 2 new rescued American white pelicans.

[52][53] The 15 in (381 mm) gauge rideable miniature railway, first opened in 1958, continues to transport guests around the zoo when weather allows.

The #82 was eventually withdrawn, due to being too small to pull the longer trains the zoo needed to handle the crowds, and was placed on display.

In 1989, the zoo loaned #82 to the recently rebuilt Riverside and Great Northern Railway, and later traded the engine to the R&GN for a second diesel, #1992.