Ellis was previously group finance director of UK bank HBoS until October last year. He was appointed as Halifax chief operating officer in 1999 and was charged with overseeing the company's merger with Bank of Scotland.
WHSmith chairman Robert Walker said: 'With his extensive financial experience, including retail banking, Mike will be a valuable member of the board as we implement our turnaround plans for the group.'
Ellis's arrival is the second big financial appointment to the WHSmith board this month, with the appointment of former Thomas Cook chief executive Alan Stewart on March 4. He replaced John Warren, who stepped down after the sale of the international business left him with a reduced role.