Brexit and the Northern Ireland Constitution considers the intersection of two processes: the complex and constitutional process of the United Kingdom's withdrawal from the European Union - Brexit - and the steady yet fragile development of the Northern Ireland constitution deriving, primarily, from the Belfast 'Good Friday' Agreement of 1998. Interdisciplinary in approach, the analysis draws on legal and political theory to develop a novel framework for assessing the progressive impact of Brexit on the Northern Ireland constitution based on systematic definitions of both. This approach elucidates dynamics and implications not yet considered in the otherwise extensive debates about Brexit and its impacts on Northern Ireland. Based on detailed analysis of the Brexit process it is argued that its impact on the constitution of Northern Ireland has been profound. Fundamentally, Brexit changed the political and legal environment in which the Northern Ireland constitution had existed for over twenty years. Embracing 'constructive ambiguity' the 1998 Agreement recognises and accommodates the concerns of both unionists and nationalists in Northern Ireland; it did not therefore solve the constitutional conflict but rather allowed it to be managed differently through an innovative system of multileveled governance: within Northern Ireland (power-sharing devolution), on the island of Ireland (North-South cooperation), and between the islands of Great Britain and Ireland (East-West cooperation) all underpinned by a multifaceted principle of constitutional, popular, and cross-community consent. By forcing a paradigmatic shift in the way that the systems of government established by the 1998 Agreement operate, Brexit disrupted the 'constructively ambiguous' compromise that it represents. Completed two years after the legal implementation of UK withdrawal from the EU, Whitten concludes by considering the potential longer-term constitutional repercussions of Brexit both within and beyond Northern Ireland's (recently notorious) borders.