The conventional image of Ice Age environments of North America includes mammoths feeding on grasses in open tundra or steppe habitats and mastodons browsing on spruce branches in forests. However, re-examination of plant and animal fossil research in the Great Lakes region of the USA and adjacent Ontario, Canada provides new insights into the changing diets of mammoths and mastodons in this region, particularly as these animals neared extinction between 13,500 and 13,000 calendar years Before Present (cal yr BP). This paper reconstructs the following scenario at the end of the Ice Age. Woolly mammoths primarily inhabited tundra adjacent to the northward receding margin of the Laurentide ice sheet. Meanwhile, to the immediate south, Jefferson mammoths grazed on grasses, sedges and herbs around the edges of wetlands, while American mastodons consumed mainly the leaves and branches of spruce and other trees in first an anomalous spruce parkland/sedge wetland environment and later in spruce-dominated forest. However, mammoth and mastodon populations began to dwindle at a time when the succeeding vegetation became a closed forest with a lesser amount of spruce trees, grasses and sedges and a greater abundance of invading deciduous trees. The last mammoths and mastodons in the Great Lakes region bear signs of stress and competition for the same foods in dense coniferous-deciduous forest, which contributed to the extinction of these magnificent beasts by ∼13,000 cal yr BP. This extinction event highlights the fragility of mammal populations under stress; an important lesson given that numerous species today are similarly challenged by climate and landscape change.