The MPM herbarium (MIL) began with a donation of 5,190 plant specimens in 1883 to the new City of Milwaukee Museum from the Wisconsin Natural History Society. This early collection has some of the oldest material in the herbarium, dating back to the 1850s and is heavily European in origin. Today the collections number around 250,000 specimens including ca. 70 type specimens with over 50% of the material from Wisconsin and another 30% from the rest of North America. The collections are divided into vascular (107,000 records) and nonvascular (17,000 records) plants with associated data digitized.

Significant special collections within the vascular herbarium include the Charles E. Monroe’s Asters, Herman Hayward’s Black Hills plants and Southeastern Wisconsin Regional Planning Commission’s inventory of natural areas. Reflecting the research interests of its curators, MIL holds collections of the Huron H. Smith’s ethnobotanical material of Wisconsin Indians, Albert Fuller’s eastern North America Rubus, Emil Kruschke’s Wisconsin Crataegus, W. Carl Taylor’s ferns and fern allies of Arkansas. The nonvascular herbarium includes J.F. Brenckle’s Fungi Dakotensis, a partial set of Richard Spruce’s Musci Amazonici et Andini and lichens collected by Wisconsin’s first scientist, Increase A. Lapham.