American mastodons were among the largest living land animals during the ice age. They ranged from Alaska and Yukon to central Mexico, and from the Pacific to Atlantic coasts.

Compared to living elephants and mammoths, American mastodons were squatter—from 2 to 3 m (7 to 10 ft.) in shoulder height—and longer—about 4.5 m (15 ft.).

Their upper tusks extended 2 m (7 ft.) or more beyond the sockets, and some mastodons had vestigial tusks in their lower jaws. The tusks were probably used for breaking off branches of conifer trees to eat.

Their cheek teeth consisted of paired, blunt cones covered with thick enamel—useful for browsing on trees and shrubs. Coarse, reddish hair has been found on the best-preserved specimens.

Their preferred habitat was open spruce woodlands, spruce forests and marsh. Their diet included conifer twigs and cones, leaves, coarse grasses, mosses and swamp plants.

In Canada, most mastodon remains (more than 60 specimens by 2008) have been found in deposits that postdate the last glaciation in southern Ontario. Fossils have been found in every province and territory except Nunavut, Newfoundland and Labrador, and Prince Edward Island.

American scimitar cats (Homotherium serum) preyed on young mastodons, and Paleoindians sometimes hunted adults. Although people may have contributed to mastodon extinction, which occurred about 9000 years ago, rapidly changing climate and plantscapes near the close of the Pleistocene Epoch seems to have been more significant.