This Article argues that the forces affecting Brexit are rooted in 19th century Britain. It deconstructs the familiar narrative that casts the US as the archetype of a popular sovereignty model with a formal supreme Constitution and judicial review. In that narrative, the UK is cast as the antithesis. This Article reveals that, even as this narrative was becoming orthodoxy during the 19th century, the UK was already operating under a model similar to the US, demonstrating a continued commitment to popular, rather than parliamentary, sovereignty. 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. The common Anglo-American model sheds new light on the meaning of the government’s mandate at elections, the rise of party power, the status of constitutional conventions and the conditions that would legitimize court packing.