Takes over as CEO from Alan Tomlin who left the company at the end of last month.
Mr Hennessy joins thetrainline.com as more and more corporates use trains for business travel and a growing number of bookings are made online.
Mr Hennessy worked at Avis Europe for four years. Before that he has worked for the John Lewis Partnership, Buy.Com (UK) Limited, Pepsico Restaurant Services and Tricon Global Restaurants.