Throughout much of its range, Tsuga canadensis (L.) Carr. (eastern hemlock) is severely threatened by the hemlock woolly adelgid (HWA; Adelges tsugae Annand), an exotic, invasive insect introduced from Japan. HWA causes defoliation and mortality, which presents a serious threat to the survival of T. canadensis in forests of eastern North America. Although HWA has been extensively studied in the forests of New England and the Mid-Atlantic, its spread into forested ecosystems throughout the southern Appalachian Mountains is relatively recent, and few studies have addressed the early impacts of this biological invasion. This study utilized forest inventory data collected in 2003, and again in 2008–2009 to quantify T. canadensis mortality and consequential shifts in species composition resulting from infestations of HWA throughout Great Smoky Mountains National Park. We observed significant decreases in the density of overstory and understory T. canadensis. The majority of the surviving overstory T. canadensis trees are in severe decline, and will likely die in the coming years. Furthermore, understory T. canadensis trees have experienced greater rates of mortality than overstory trees. No individual site or stand characteristic was correlated with mortality. Overstory and understory species composition did not significantly change over the observation period, but the overwhelming presence of Rhododendron maximum L. (rosebay rhododendron) in the understory may influence future successional trends in forests of the southern Appalachian Mountains.