Walsh has joined APCO as regional director, EMEA, after nine years at IBM.
IBM declined to comment on whether it was replacing Walsh in the Paris-based role, confirming only that the position was now vacant.
Walsh will manage the agency's day-to-day European operations at APCO, while advising IT and telecoms clients and bringing in new business.
Based in London, he plans to spend two days a week in Brussels working for clients such as Microsoft and the Business Software Alliance (BSA).
He reports to Brad Staples, APCO's chief executive across EMEA.
In his nine years at IBM, Walsh held various positions including the handling of comms surrounding the integration of PwC Consulting in Europe, following its acquisition by IBM in 2002.
In the first five years of his career he was a parliamentary assistant in the House of Commons. From 1994-1996 he worked for lobbying outfit Ian Greer Associates in London and Brussels.
Walsh is the second major name in European comms that APCO has lured in the past few weeks. Jon Chandler, EU director of strategic communication for Coca-Cola, quit to join the agency's corporate comms team as regional director corporate comms EMEA last month (PRWeek, 11 August).
IBM has blamed a slowdown in spending by corporate and government buyers for a flat performance this year. But it has recently been on an acquisition spree.
In August the firm bought MRO Software for £389m, paid £850m for Californian software developer FileNet, and £740m for Internet Security Systems.
IBM this month celebrated the 50th birthday of the hard-disk drive.