Increasing the extent of irrigation in sub-Saharan Africa is a potential approach to closing yield gaps and improving resilience to climate change. Using over 3,000 satellite images spanning more than 30 years, we show that development of irrigation infrastructure in the Senegal River Valley has led to a large increase in average cultivation rates: a sixfold increase from 4 to 24 percentage points in the non-rainy season and a tripling from 10 to 30 percentage points cultivated in the rainy season. In spite of this substantial increase in cultivation rates, we find widespread heterogeneity across projects, with 1/4 of total irrigated area reamining unused as of 2019. We provide farmer survey evidence that limited access to water remains a major constraint on production and that removing this constraint requires action beyond individual farmers.