He joined Yahoo! in late 2007 and this June was handed a promotion by recently appointed Yahoo! chief executive Carol Bartz.
Fahy, formerly vice-president of marketing for Europe, took the new role of vice-president of international marketing.
Fahy took on responsibility for Yahoo!'s brand and consumer marketing and communications activities for all countries outside the US as the business wants to reposition to compete better with Google and Microsoft and maximise revenue potential in Europe and global emerging markets.
At William Hill he takes up a new senior management position of brand and marketing director, reporting to chief executive Ralph Topping.
He will be responsible for development and control of the William Hill brand across all the channels in which the business operates.
He has previously worked in marketing roles at Orange and Blackberry.