Hannan was the first secretary-general of the Alliance of Conservatives and Reformists in Europe (ACRE), serving from 2009 to 2018. He was one of the founders of Vote Leave, one of the organisations that campaigned to leave the EU in 2016, and served on its board throughout the referendum. He played a prominent role in the referendum campaign, participating in a number of public debates. He stood down from the European Parliament at the United Kingdom's exit from the EU in 2020.