Helen Mountfield, Q.C., is a barrister in England, where she is a founder member of Matrix Chambers, which has a strong international presence. She specializes in constitutional and administrative law and has appeared in many leading cases in the Supreme Court, the Court of Justice of the European Union and the European Court of Human Rights. These include being leading counsel for the Pigney group of claimants in Gina Miller v Secretary of State for Exiting the European Union. Helen is also an accredited mediator and sits as a deputy High Court judge. From September 2018, she will also be principal of Mansfield College, Oxford.