His primary areas of research and teaching include environmental history, North American borders, and Latino history. He has taught courses on North American and world environmental history, natural disasters, immigration and ethnicity in the United States, and border and transnational history more generally. His first book, Revolution in Texas: How a Forgotten Rebellion and Its Bloody Suppression Turned Mexicans into Americans (Yale University Press, 2003) offered a new interpretation of the origins of the Mexican-American civil rights movement. Johnson’s other primary interest is in the social and political history of American environmentalism, the subject of his current book “Escaping the Dark, Gray City:” How Conservation and the Arc of the Progressive State.