Mercer is expected to start a full-scale review of Eurostar's pricing structure and oversee marketing activity. Marketing director Bertrand Guillon will report to Mercer.
Mercer launched Air Miles as its marketing director in 1988. He has since been business development director at First Direct and marketing director at interactive TV service Open. He was most recently marketing director at software firm Misys.
Eurostar's passenger numbers fell by 3.3% for the fifth consecutive quarter in the three months to September 2002.
It has recently offered special fares from £49 return to Buzz passengers whose flights have been cancelled due to the proposed Ryanair takeover.
Its latest TV ads, by TBWA/London, star Mark Gatiss from TV series League of Gentlemen.