The following paper analyzes the political theory of Neil MacCormick, a prominent figure of recent political and legal philosophy and a member of the of the European Parliament from 1999 to 2004. The article falls into three parts. The first one examines the main features of his ‘Institutional Theory of Law’ as a normative construction of commonly accepted social issues. The second part investigates his idea of sovereignty in its main aspects (political and legal, internal and external), deepening further on the differences between Rechstaat and Polizeistaat and on the operative issues of the rule of law in a constitutional perspective. A third and final part will try to ponder MacCormick's main thesis in the light of the United Kingdom's withdrawal from the European Union in June 2016 (so-called ‘brexit’).