M. decora is a parasite of vertebrates, including humans, and an aquatic predator of eggs, larvae, and other invertebrates.

M. decora, like all leeches, is hermaphroditic, and has ten testisacs and two ovisacs, in addition to male and female genital pores.

Macrobdella decora was originally placed in the genus Hirudo by Thomas Say, who described it in 1824 in an appendix to a book about an expedition up the Minnesota River.

The stomach, a large pouch composed of smaller sacs, is not nearly as muscular as the pharynx, but it occupies about five sixths of the leech's whole body and is subdivided into eleven chambers.

The leech's four copulatory glands are arrayed in a square in an area of rough skin on segments thirteen and fourteen.

[13]: 509 The most widely distributed Macrobdella species, M. decora, is found in North America east of the Rocky Mountains in southern Canada and the neighbouring United States.

[19][20] Macrobdella decora is a freshwater species that is found in still or slow-moving water bodies such as streams, temporary ponds, ditches, and wetlands.

It sucks the blood of many vertebrates, using its teeth to pierce the host's skin, including humans but also amphibians, fish, turtles, wading birds, and cattle.

It also hunts voraciously, and eats oligochaete worms, snails, amphibian eggs, the larvae of insects, and even other individuals of its own species.

[23] The gut microbiome of the North American Macrobdella decora is quite similar to that of Europe's Hirudo verbana.

[4]: 5 Macrobdella decora does parasitize humans and is often found by people swimming in Canada and the northern United States.

[3]: 132  It is possible that human leech-trading helped move leeches between water bodies; today, they are sometimes used as and transported for fishing bait, but they are not a very favoured choice.