The objectives of the present study were to explain the origin and to reconstitute the different phases of the paleovegetation of the Keswick marsh, Ontario. Palynological evidence was used to key out the regional and extra-local paleovegetation, and macrofossils analysis served to establish the history of the vegetation at the field site. The data showed that in the vicinity of Lake Simcoe the origin of the marsh was closely associated with the changes in the climatic conditions that favored the development of an environment suitable for peat-forming plant communities; 2.4 m of peat accumulated in the last 4000 years on a washed silty clay. The macrofossils of hardwood species, seeds of Aralia racemosa, and sporangia of Polypodiaceae at the base of the deposit indicated that a mixed forest, composed mainly of deciduous species surrounded by tamaracks, prevailed prior to the initiation of peat accumulation. Furthermore, additional vegetation changes occurred, notably the replacement of an arboreal swamp vegetation by an herbaceous marsh, followed by a shrub–herb marsh and then an herbaceous marsh, recently drained and cultivated.