This is the first detailed description of the remains of a young adult American mastodon (Mammut americanum) found in 1936 in peaty silt and clay underlying till in gypsum karst near Hillsborough, New Brunswick. It documents 312 fossils comprising a partial skull with molar teeth and tusks, a neck vertebra, and much of the right postcranial skeleton. An individual age of 15 – 18 years and a weight of 8.3 t are estimated for this mastodon. Associated spheroids, containing cut wood fragments and an unusually high clay content, are interpreted to be mastodon coprolites. Radiocarbon ages are 13 600 ± 200 (bone), 37 200 ± 1 310 (coprolite wood), 51 500 ± 1 270 (coprolite carbonate cement), and >43 000 BP (peat). Pollen in the coprolites and associated sediment indicates a coniferous forest. Nine other mastodon fossils from Nova Scotia include a femur from Middle River, which dates 31 300 ± 500 BP and contains pollen representing boreal forest – tundra, and three molars from offshore Georges Bank. All ages are judged minimal: the older four are at or near the limit of the method; the younger is likely incorrect because of preservative contamination. Associated pollen assemblages correlate with late last interglacial age deposits in the region and differ from possible Middle Wisconsinan age deposits. The fossils are regarded as a single group and are assigned to a cool phase of the Sangamonian interglaciation, probably oxygen-isotope substage 5a, prior to Wisconsinan glaciation.