He will be succeeded by global head of executive comms Ed Williams.
Williams will take the title director of corporate affairs from 2 April, reporting to Reuters CEO Tom Glocer.
Walker is to become a full-time senior adviser to Glocer and his chairman Niall Fitzgerald.
Walker - who was put on the Conservative Party's 100 strong ‘A-list' of priority candidates last year - said the move would allow him to ‘focus more on seeking a political career', adding that he would stay with Reuters for ‘another year or so'.
Walker was previously comms chief at Buckingham Palace and British Airways.
Williams joined last January after a five-month stint as director of corporate comms at mobile phone firm 3. Prior to that he spent more than five years working as a director at Brunswick, and four years as a senior news editor at GMTV.
He has also worked as a PR adviser to former US general Wesley Clark.
Williams said the change had been ‘on the cards' since he joined, and was part of ‘deliberate succession planning on Simon's part'.