In 1983, Ernst became an attorney alongside the eponymous Paul de Clercq, with whom he went on to form the partnership De Clercq and Van Win Lawyers in 1987. He became a registered and certified business mediator in 1996. Ernst was one of the initiators of the Grotius Employment Law specialist course and has lectured in this, as well as in rules of conduct at the Professional Law School. He joined the Supervisory Council in The Hague in 2006, in the role of ‘Leids Deken’ (Dean of Leiden). In 2009, he was elected as ‘Haags Deken’ (Dean of The Hague) and held this supervisory and administrative position until 2012. He has been a member of the Lawyers’ Conduct Review Committee, and performs various administrative and supervisory roles in the art, museum and healthcare sectors.