To help ground agri-environmental policy in evidence, long-term trends in agri-environmental and agricultural land use data in Ontario, Canada (1826-2011, 1921-2011) were identified using correlation and trend detection (Mann-Kendal test) and compared to other parts of Canada and other developed countries. After historic expansion 1826-1931, farm area decreased 1931-2011 by 4 million ha or 44.5%, similar to decreases in eastern Canada, eastern US and other developed countries, but contrasting continued but slowing expansion in western Canada. Ontario crop area varied little 1921-2011, but with large shifts in crop types and production increases. Crop area in eastern Canada was declining or stable, while in western Canada continued increasing. Major changes 1921-2011 included significant declining pasture area (-78.7%), hay area (-44.9%) and cattle numbers (-33.9%), along with increases in soybean and corn area and in chicken and hog numbers-transforming landscapes, similar to trends in eastern Canada but contrasting western Canada. Two periods of change are identified using principal components analysis, 1921-1970s, a long modernization period (69.1% of variance) and the 1970s-2011 period (20.1% of variance), with decreases in cattle numbers, hay and pasture and increases in hogs, chickens, soybeans, corn and wheat. After the 1970s, agri-environmental stewardship increased, and some environmental risks decreased with reduced pesticide use (-45%, 1983-2008), phosphorus fertilizer use (-30.1%, 1981-2011) and estimated manure volumes (-42.6%, 1976-2011). Illustrating the regional diversity of agricultural change, Ontario trends are unique, differing considerably from western Canada, but having some similarities with other eastern provinces, the mid-west and eastern US and many other developed countries. The research emphasizes the need to base agri-environmental policy in sound empirical trend analysis.