James is married to Marzenna Kowalik (1964 – ), a political scientist who specialises in Polish-Soviet economic relations who also teaches at Princeton. They have three children.
In the earlier part of his career, James focused on modern German history, particularly German financial history in the interwar era. Among his major contributions to the field are a detailed study of Deutsche Bank, an examination of the role of the Reichsbank in seizing Jewish financial assets during the Nazi era, and a reappraisal of the peculiar nature of the German national identity. James' explanation of the evolution of the German identity places particular emphasis on an "economic identity", which provided the impetus for unification in the 19th century. In 1992 he was appointed to the Independent Commission of Experts, which had been set up by the Swiss Parliament to examine the refugee policy of Switzerland during World War II as well as economic and financial relationships between Switzerland and Nazi Germany.
Harold James has written extensively on the economic implications of globalisation, drawing comparisons with historical attempts at globalisation that ended with the Great Depression beginning in 1929. He argues that the Great Depression must not be considered as only an American phenomenon, but as a global economic crisis. He examines the contemporary issues associated with globalisation in the context of larger economic trends, which were disrupted by the World Wars and the Great Depression.
Interwar Depression in an International Context (2002) ISBN3-486-56610-5
Europe Reborn: A History 1914–2000 (Pearson Longman, 2003) ISBN0-582-21533-1
International Financial History in the Twentieth Century (ed.) (with Marc Flandreau and Carl-Ludwig Holtfrerich) (Cambridge University Press, 2003) ISBN0-521-81995-4
Nazi Dictatorship and the Deutsche Bank (Cambridge University Press, 2004) ISBN0-521-83874-6