Old-growth forests often represent an important natural benchmark for evaluating the effects of management and changing environmental conditions on forest and ecological dynamics. In regions with a history of extensive land use, such as New England, there is limited opportunity to develop such information, given that only a few old-growth forests remain scattered across a finite portion of the landscape. This study takes advantage of a unique historic data set collected across 250 ha of old-growth forest, within a larger area (2,000 ha), in southern New Hampshire in 1929–30, to characterize natural forest composition, live-tree size distributions, recruitment history, and aboveground biomass conditions across six common regional forest types. Forest composition ranged from areas dominated by Pinus strobus L. and Tsuga canadensis (L.) Carrière to mixed hardwood forests composed of Fagus grandifolia Ehrh., Betula lenta L., and Betula papyrifera Marshall, and Acer saccharum Marshall. Across forest types, smaller diameter classes were composed primarily of shade-tolerant species, namely T. canadensis and F. grandifolia, whereas larger size classes consisted of P. strobus and T. canadensis, with the largest size class composed of P. strobus exclusively. Age data collected from these forests reflected largely multicohort conditions, with the largest peaks in recruitment documented between the mid-1600s and mid-1700s. Biomass values were positively correlated with stem density, and stratified forests containing high densities of smaller diameter T. canadensis in both the understory and midstory as well as P. strobus in the overstory tended to have the highest basal area and a greater number of trees within the largest diameter classes (≥ 89 cm). Forests with high basal area densities of large P. strobus (≥ 51 cm) had the greatest biomass values. In these sites, understory T. canadensis stem density was relatively high, likely suggesting that the stratification of both species accounted for the high biomass values; however, the presence of large-diameter P. strobus was most important to generating those high biomass totals. The wide range of composition and structure observed across this landscape likely reflects the historic importance of variability in disturbance processes and site conditions in maintaining a mosaic of forest compositional and structural conditions and may serve as a reference for restoration and conservation activities in areas heavily affected by past land use. Moreover, the disproportionate influence of P. strobus on aboveground biomass conditions in areas in which it historically predominated highlights the unique potential of this species for use in regional mitigation strategies.