We examined historical changes and patterns in the nonnative flora of Franklin County in western Massachusetts. The historical flora was documented by examining more than 16,000 herbarium specimens and additional records from the literature. We characterized the recent flora during 10 years of field work from 2010 to 2019. Temporal changes and distributions of nonnative species were examined at the town level. Among all species recorded in the county, nonnatives comprised 27.4%, or 34.8% if waifs and species persisting from cultivation with little spread are included. Nonnative species as a group increased dramatically over time relative to native species, and those nonnatives considered invasive increased more than other nonnative species. Reported frequencies of several species increased from 0 to 2 towns in the historical flora (1800–2009) to most of the 26 towns in the county today. The proportion of nonnative species in a town's flora was correlated with the human population density, but different regions of the county did not differ significantly in the proportion of nonnative species. Species from eastern Asia showed greater increases in frequencies than species from Eurasia and parts of North America outside New England, and the Asian species had later average arrival dates in the county. Species with bird-dispersed seeds were better represented among taxa showing the greatest increases than were species dispersed by wind or other means. Nonnative species with a history of herbal or culinary use increased much less than those with other patterns of human use, presumably reflecting less emphasis on some such species in local gardens. As a group, the agricultural weeds did not show patterns of change different from other species, perhaps reflecting the continued importance of agriculture in parts of the county.