Until the most recent continental ice sheet retreated 11 000 years ago, the Mixedwood Plains Ecozone was buried under more than a kilometre of ice. Following the ice age, the St. Lawrence and lower Ottawa valleys were covered by the Champlain Sea for another 1 200 years. Thick marine clay deposits in southern and southeastern Ontario and southwestern Quebec are the products of glacial action and the now extinct Champlain Sea. Beneath the urban centres and agricultural fields are mesozoic and paleozoic sedimentary rock.

Striking physical features break up the otherwise nearly level to gently rolling plains of most of the ecozone. One of the most prominent is the Niagara Escarpment from Niagara Falls to the northern tip of the Bruce Peninsula and Manitoulin Island.

Limestone, shale, and sandstone are characteristic of the flat-lying St. Lawrence lowlands. Mount Royal rises to 227 metres above sea level from the widest and flattest sections of the Montreal Plain. Some visually dominant landforms in the area are the St. Narcisse terminal moraine on the north side of the St. Lawrence River, and the Drummondville and Highland Front moraines of the south shore. Over 6 000 drumlins are scattered among the extensive sand and limestone plains in the ecozone's southern stretches between lakes Huron, Ontario, and Erie.

The Mixedwood Plains Ecozone is endowed with abundant freshwater resources, including four of the Great Lakes -- Superior, Huron, Erie, and Ontario -- and the St. Lawrence River from Kingston to Quebec City. These Great Lakes constitute nearly 20% of the world's fresh water. The outflow of the St. Lawrence is the greatest of any river in Canada and ranks thirteenth worldwide. Tributaries, such as the Ottawa, Maurice, and Saguenay, bolster the river's flow along its 600-kilometre route to the gulf. Rivers and lakes occupy roughly 42% of the ecozone's total surface cover.

Variations in climate, vegetation, and soils influence the land-use patterns. Podzol soils in northern stretches, while generally useless for agriculture, are suited to forestry and recreation. In the south, grey-brown Luvisol soils, developed under forest vegetation from glacial deposits, are favoured for agricultural crops such as tobacco and fruit.

The climate of the Mixedwood Plains produces relatively warm summers and cool winters moderated by surrounding water bodies. Mean daily January temperatures range from -3°C to -12°C, whereas mean daily July temperatures are 18°C to 22°C. The ecozone supports a wide variety of agricultural activities with an average annual growing season, north to south, ranging from 1 750 to 2 500 growing degree days above 5°C. This region also receives 720 to 1 000 mm of precipitation annually. Due to its location in the midst of a significant North American storm belt, weather in the Mixedwood Plains can change rapidly. Several southern cities, such as Woodstock and Guelph, receive considerable amounts of snow.