Trump and Brexit are at the forefront of political discussions around the world. Many treat them as symptoms of the same phenomenon: the rise of populism, nationalism, and xenophobia towards immigrants and refugees. Both seem to repeatedly challenge constitutional limits on a variety of fronts. Brexit was approved in a referendum by a slim majority, with wavering public support and in spite of a reluctant Parliament. Yet, all British political players feel bound by its results and have taken steps to withdraw from the EU, absorbing the costs of trillions of dollars to their economy. Exclusionary policies may not be enough to explain the extraordinary politics involved. This Article argues that the forces affecting Brexit are rooted in nineteenth century Britain. It deconstructs the familiar narrative that casts the US as the archetype of a constitutional model, with a formal supreme Constitution, judicial review, and popular sovereignty. In that narrative, the UK is cast as the antithesis, because Parliament reigns supreme, it has no formal Constitution, and it lacks a doctrine of judicial review. This Article reveals that, even as this narrative was becoming orthodoxy during the nineteenth century, the UK was already operating under a model similar to the US, demonstrating a continued commitment to popular, rather than parliamentary, sovereignty. The fact that Parliament refers major decisions to the People and carries out those decisions, as exemplified in the British determination to go ahead with Brexit, signals that the People is the sovereign, not Parliament. The challenges encountering popular sovereignty have remained the same over the past two centuries though gaining new dimensions: enfranchisement, protectionism, territorial divisions, and allocation of legislative power. This Article demonstrates how Britain has been operating under a common Anglo-American constitutional model for the past 200 years and highlights its implications for comparative constitutional law. The common Anglo-American model sheds new light on the meaning of the government’s mandate at elections, the rise of party power, and the conditions that would legitimize packing the courts.