The southernmost site where permafrost has been located in the Quebec–Labrador peninsula is in a peatland on the subalpine summit of Lac des Cygnes Mountain (47°41′N, 70°36′W). Because of the thickness of its peat deposit, this site contains rich ecological information about the history of the peatland and its surrounding subalpine environment. We conducted a detailed macrofossil analysis to reconstruct the 6000-year history of the peatland. In general, the development of the Lac des Cygnes Mountain peatland has followed the classic succession of rich fen – poor fen – bog found in other peatlands in subarctic, boreal, and temperate environments. An abrupt decline in all tree macrofossils ca. 4000 BP suggests that a major deforestation event occurred on the summit of Lac des Cygnes Mountain. The exact cause of this deforestation event is unknown, but fire is a possible factor. The lack of absolute chronological resolution near the top of the macrofossil record precludes proper dating of permafrost inception, but the balance of evidence appears to indicate that the permafrost is modern.