There is little variation in the climate across the Mixed Wood Plains. Summers are warm, with mean temperatures ranging from 16.5°C around Quebec City to 18°C in the lowlands north of Lake Erie. The range in winter mean temperatures is wider: -2.5°C in the extreme southwest to -7°C in the easternmost part. Mean annual precipitation is lowest (720 millimetres) around the juncture of the St. Lawrence and Lake Ontario, and highest on the St. Lawrence Lowlands, at 1000 millimetres. The weather, however, can be very volatile, especially in southern Ontario, which has the highest annual frequency of thunderstorms and tornadoes in all of Canada.

About 15 million people live in the Mixed Woods Plains ecozone, the most densely populated region in Canada. Canada’s two largest cities, Toronto and Montreal, are located here, as are other large urban centres like Ottawa-Hull, Quebec City, and

Hamilton. Although agriculture is important, most of the population works in the service and manufacturing sectors. The Mixed Woods ecozone has four National Parks: Bruce Peninsula, St. Lawrence Islands, Point Pelee, and Fathom Five. The last of these, located in and around the deep, clear waters of Georgian Bay, is Canada's first National Marine Conservation Area.