Analysis of nationally representative individual-level panel data from 1980 to 2010 reveals a significant negative trend in the agricultural labor supply from rural Mexico, which is the primary source of hired workers for U.S. farms. These findings offer an explanation for the rise over time in U.S. farm wages. Concomitants of the agricultural transformation, including growth in the non-farm economy, falling birth rates, and an increase in rural education, accelerate the transition of rural Mexicans out of farm work. Higher U.S. farm wages and increased border enforcement slow the transition, but the combined impact of these offsetting variables is relatively small. A diminishing farm labor supply has far-reaching implications for farmers, farm labor organizers, rural communities, and agricultural workers.