Soils used for crop production cover 15.5 million km2 and almost all have been tilled at some point in their history. However, how the changes in soil depth and soil properties associated with tillage affect crop yields is unclear. Here, we show that tillage on slopes thins soils and reduces wheat and maize yields. At the landscape scale, tillage erosion gradually reduces crop yields as the duration and intensity of tillage increase. Over the next fifty to 100 years, the overall yields are likely to further decline as modern mechanized agriculture accelerates the process of tillage erosion compared to centuries of non-mechanized tillage. Arresting this downward trend will require more widespread adoption of no-tillage practices and avoidance of downslope cultivation. The downward pressure on landscape-scale yields due to tillage erosion is expected to be amplified by climate change-induced increases in dry spells during crop growth.