Wehler was born in Freudenberg, Westphalia. He studied history and sociology in Cologne, Bonn and, on a Fulbright scholarship, at Ohio University in the United States; working for six months as a welder and a truck driver in Los Angeles. He took his PhD in 1960 under Theodor Schieder at the University of Cologne. His dissertation examined social democracy and the nation state and the question of nationality in Germany between 1840 and 1914. His postdoctoral thesis on Bismarck and imperialism, opened the way for an academic career. His habilitation project on "American imperialism between 1865 and 1900", supported by the American Council of Learned Societies, permitted him to do research in American libraries in 1962–1953 and resulted in two books. In all he spent six years in the US and was strongly influenced by its academic structures and by research in comparative modernization.