London Mayor Boris Johnson has hired the BBC's Westminster news editor as his new communications chief.
He replaces another former BBC journalist, Guto Harri, who left to join News International following Johnson's successful re-election.
Walden began his career as a reporter in local newspapers before moving to commercial radio and has since worked for as a reporter, producer and editor for the BBC, ITN, GMTV and Granada.
He has been in his current role for the past five years, overseeing the BBC's day-to-day coverage of political news.
He said: 'After 12 fantastic years at the BBC, I'm delighted to be joining Boris Johnson and his team at what is an incredible time for London and for Londoners."
Johnson said Walden would bring 'drive, originality and flair to the post of director of communications".
"It is ever more vital that politicians communicate clearly and honestly with voters about how we are spending their money and how we plan to improve the city,'he added.
"I believe Will has exactly the right skills to help us engage with Londoners. He has enormous experience of politics and a great track record at the BBC. I am thrilled to welcome him on board."
Walden's appointment comes just two weeks after Johnson launched a strongly-worded attack on the BBC, claiming it was 'overwhelmingly biased to the Left'and that the new director-general should be a Tory.
In his weekly column for The Daily Telegraph, Johnson claimed his 'chief opponent'in his recent election victory was the local BBC news, adding: 'The prevailing view of Beeb newsrooms is, with honourable exceptions, statist, corporatist, defeatist, anti-business, Europhile and, above all, overwhelmingly biased to the Left."