In 1604, the Elizabethan Society of Antiquaries examined three components of King James’s proposed Anglo-Scottish union: the unity of name, law, and Parliament. As members of the Society reconstructed English history in their papers, a variety of historical and constitutional positions emerged concerning English institutions and national identity. This article argues that these various renderings of English ancient constitutionalism reveal a protean and heightened sense of England’s juridical past. It contextualizes these divergent narratives in the identity crisis engendered by the proposed Anglo-Scottish union and gives special attention to those antiquaries who saw the law and Parliament as a more recent and complicated development. In doing so, these antiquaries portrayed a constitution that was neither ancient nor immutable, but rather one that was constructed from a mishmash of foreign customs. By analyzing the antiquaries’ arguments, this article seeks to broaden the historiographical understanding of ancient constitutionalism and to show that the antiquaries made a significant contribution to early Stuart political thought.