This is a comprehensive study of extreme weather events in southern Ontario from 1950 to 2017, and their impacts on winter wheat and oat yields. Trends in temperature and precipitation were evaluated annually and seasonally. There were significant shifts toward increased warming, growing season length, and the frequency of precipitation events. Warm and precipitation extremes are increasing in intensity, duration, and magnitude. Random Forest regression was used to investigate how different extreme weather indices were related to winter wheat crop yield, across crop pheno-phases and controlling for soil texture. Crop-specific indices were important indicators, explaining 40% of yield variance. Winter Warming Index was the most important index in the RF model, linked to a 72% increase in mean square error when removed. Changing extreme weather distributions in southern Ontario seems to be increasing potential negative impacts on farming winter wheat and milling oats, so adaptive plans should be considered.