Phil Georgiadis, her founding partner, will take over leadership of the business when she leaves at the end of June.
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.