Mr Harri was previously linked with a job at Tory central office
New London mayor Boris Johnson has hired ex-BBC political correspondent Guto Harri as his director of communications.
Mr Harri, 41, left the BBC earlier this year to work as a consultant for PR company Fleishman Hillard.
But he has now resigned his post to take up his new role at City Hall.
Mr Harri, who was at Oxford University with Mr Johnson, had previously been linked with a job as a spokesman for Tory leader David Cameron.
Mr Harri was at Mr Johnson's side earlier as he held talks with New York mayor Michael Bloomberg at City Hall.
He told BBC Wales: "I have already been in a meeting with Boris and Mayor Bloomberg and there was a lot for Boris to hear that was interesting in relation to getting crime down and getting the office of Mayor more in touch with people.
"I spoke to Boris before the election and he asked me then if I was interested in the job with him - then he rang me after he was elected.
"I had to go through an independent review panel at City Hall, but after that I resigned from Fleishman last night (Thursday)."
Conservative MP Mr Johnson is famed for his supposed gaffes, and he recently said the media had a "pent-up rage" after spending the mayoral campaign "deprived of their prey - a Johnson blooper".
But Mr Harri said his role would not be to keep the new mayor in check.
"It will be fascinating - but it won't be about containing Boris - he is a very serious man," he told BBC Wales.
He earlier told Iain Dale's blog he had been in Zimbabwe for the past four weeks advising Movement for Democratic Change leader Morgan Tsvangirai.
"If I can handle Zimbabwean politics I should be able to handle Boris Johnson," he told the blog.
Mr Harri was BBC Wales' political correspondent, before moving to London to cover the UK and the world for the Welsh language news service provided to S4C by the BBC.
He joined BBC News 24 as Chief Political Correspondent in November 2002.
He was then based in Rome from July 2004 to January 2005 and spent two years as North America business correspondent before returning to Westminster last year.
Mr Johnson has pledged to cut the number of marketing and publicity staff employed by the mayor's office and has already scrapped previous mayor Ken Livingstone's newspaper The Londoner.