Significance In recent years, tropical species have expanded poleward into temperate regions. For example, along the east coast of North America, mangroves have expanded into salt marshes in response to decreases in the frequency of extreme freezes. But questions remain about how mangrove abundance has changed over longer timescales and the role of anthropogenic climate change. We used a mixed methods approach to document a series of climate-driven shifts in mangrove abundance over the past 250 y. However, climate model projections suggest warming may push this fluctuating system toward a persistent state of mangrove dominance. This historical approach can be applied to a variety of ecosystems to place the effects of climate change in the context of long-term natural climate variability. Abstract Climate change is driving the tropicalization of temperate ecosystems by shifting the range edges of numerous species poleward. Over the past few decades, mangroves have rapidly displaced salt marshes near multiple poleward mangrove range limits, including in northeast Florida. It is uncertain whether such mangrove expansions are due to anthropogenic climate change or natural climate variability. We combined historical accounts from books, personal journals, scientific articles, logbooks, photographs, and maps with climate data to show that the current ecotone between mangroves and salt marshes in northeast Florida has shifted between mangrove and salt marsh dominance at least 6 times between the late 1700s and 2017 due to decadal-scale fluctuations in the frequency and intensity of extreme cold events. Model projections of daily minimum temperature from 2000 through 2100 indicate an increase in annual minimum temperature by 0.5 °C/decade. Thus, although recent mangrove range expansion should indeed be placed into a broader historical context of an oscillating system, climate projections suggest that the recent trend may represent a more permanent regime shift due to the effects of climate change.