Lee Cain donned the Daily Mirror’s chicken costume and to poke fun at the then Tory leader in the run-up to the 2010 general election.
The former Mirror journalist is now at the heart of Mr Johnson’s Downing Street operation after serving as his press aid during his tenure in Foreign Office.
Mr Cain had to contend with the chicken following around his own boss during the Tory leadership contest.
One former colleague told the newspaper that Mr Cain “attacked the role with real zeal and great passion”.
The unnamed ex-colleague said: “The newsdesk were so impressed with his work he was used on a number of occasions.
“I vividly remember him coming to the newsroom and prancing around still in his full outfit like a rooster.
“It’s hard to believe a man with his past of taunting the cowardly Tories is now such a powerful figure inside No10.”
Digital, Culture, Media and Sport Minister Nigel Adams said on Twitter: “Pleased to hear the Mirror chicken has flown the coop, abandoned Corbyn’s Labour Party and is now fully behind the PM.”
Mr Cain is not the first former Mirror journalist to become a prime minister’s spin doctor.
The paper’s former political editor Alastair Campbell was a powerful figure during Tony Blair’s time in office.
He was expelled from Labour in May after admitting voting Liberal Democrat in the European elections because of his own party’s stance on Brexit.
He has now said he does not wish to be part of the Labour Party under Jeremy Corbyn.