Walker's remaining 14 per cent stake in Walker Media is expected to be acquired by M&C Saatchi, which currently owns 75 per cent of the media agency. She will remain a shareholder of M&C Saatchi plc.
Walker and Georgiadis founded the business in 1997 with M&C Saatchi taking a 50 per cent stake. The agency group increased its stake to 75 per cent in 2004. Walker was paid an initial £3 million as part of the deal with a further payment made earlier this year.
She said: "The management team, led by Phil Georgiadis, is well placed to continue the amazing growth we have enjoyed. I will leave in the summer and take some time out abroad before I return to the UK to pursue other interests."
Georgiadis said: "I will of course miss her but I am also oincredibly excited about the company's future and my role within it."
Walker Media is now the UK's eighth largest media agency with billings above £300 million.