She replaces Mark Moorton, who left Mercer in July 2013.
Martin has previously held the roles of Europe, Middle East and Africa (EMEA) HR director and global HR leader for Mercer’s retirement and investments businesses, based in London.
Fiona Dunsire, UK chief executive at Mercer, said: “ Siobhan Martin has exceptionally well-rounded international business and academic experience and we are very pleased to have her as part of our increasingly diverse organisation.”