Clark began as a leading revisionist historian of seventeenth- and eighteenth century British history. He is notable for arguing against both the Marxist and Whiggish interpretations of the late seventeenth and eighteenth centuries. Instead, Clark emphasizes the unities and coherences of the period between 1660 and 1832. It was he who dubbed it the “long eighteenth century”, a periodisation which is now widely accepted in historical academia. Clark maintains the period was one of Anglican-aristocratic hegemony, marked by popular acceptance of the monarchy and the Church of England as symbols of national unity. This edifice was characterized by the dominance of an aristocratic-gentry oligarchy and a sense of national identity (preceding 19th century nationalism), that was firmly underpinned by a shared history and religious allegiance. In Clark’s model, Britons embraced the official entrenchment of these parameters, despite the occasional outcropping of religious dissent.
Clark has also framed an explanation of the American Revolution as, in part, a "war of religion", triggered by the denominational conflicts still endemic at that time within the English-speaking North Atlantic world.
Clark has often maintained that too often the eighteenth century has been reinterpreted in the light of the nineteenth; he sees his mission as an historian to explain the long eighteenth century in its own terms. Clark criticised earlier British historians, especially Marxists such as Christopher Hill, Eric Hobsbawm and E.P. Thompson.
He is now primarily interested in the history of religion, and his chief achievement is the reintroduction of a religious dimension into the agendas formerly set by positivist, functionalist and reductionist historians.
