An economic demographer, her primary research focuses on the causal relationship between migration and mortality, especially for migrants out of rural areas in the United States. She also works on estimating undercount and other measurement issues in the U.S. Census using demographic methods, showing the implications these problems have for economic and social research using Census data. Other research has focused on the determinants of participation spell length for Supplementary Nutrition Assistance Program (SNAP) participants and measurement of poverty in rural areas in the United States. Johnson is also highly trained in the various econometric methods used in the evaluation of the impacts of programs and policies, and recently travelled to Africa to teach impact evaluation at the request of the World Bank.