The Grandville mastodont, from Kent County, Michigan, has been radiocarbon dated at 11,320 ± 140 B.P. Longitudinal sectioning of its right tusk provides access to samples of tusk dentin formed throughout life and exposes incremental features that allow direct correlation of annual dentin increments among samples. Enumeration of annual increments indicates that 33 years are recorded in the tusk, providing an estimate of age at death. Mammut americanum typically shows well developed sexual dimorphism in tusk and body size, and the Grandville mastodont was clearly a male. A short-term decline in the rate of increase of tusk length around age 15 may reflect expulsion from a matriarchal family unit at the onset of sexual maturity. Microscopic analysis of subannual incremental laminae near the tusk pulp cavity indicates that this animal died in mid-autumn, at the end of the fifteenth fortnight following the most recent winter-spring boundary. The age and life history data obtained from this individual will be useful in characterizing the population dynamics of late Pleistocene mastodonts of the Great Lakes region and in evaluating hypotheses for their extinction.