Nick Rogers, a Conservative on the London Assembly, declared in April that he would be running to be Sadiq Khan’s opponent in next year’s election.
But on Friday, he revealed he is withdrawing his bid and backing Paul Scully MP, the Minister for London.
The announcement was made just one day after Mr Scully - MP for Sutton and Cheam - formally launched his campaign.
Mr Rogers said: “I’ve had great fun over the last few weeks talking about my ideas for the future of our amazing city.
“However, as the process has unfolded, it’s become clear that now is not my time.
“I am therefore withdrawing my name from contention and I’m delighted to say that I am endorsing Paul Scully to become the Conservative party’s mayoral candidate.
“Paul has the energy, ideas and experience to win for the Conservatives in London and I look forward to supporting his campaign.”
He added: “My sincere thanks to all those people who have been so enthusiastic about my candidacy - it has been humbling and I will be forever grateful for your support.”
Mr Rogers - who represents the boroughs of Hounslow, Kingston and Richmond on the Assembly - had said while mulling his bid earlier this year that he would “stand up for millennial Londoners”, and bring “relevant, real-world experience” to the role, having previously worked on the railways and as a special constable in the Met Police.
On Twitter, Mr Scully thanked the former candidate, saying: “Your appeal for younger Londoners to be able to realise their aspirations will be at the heart of our plans for London. Let's do this together.
“In the meantime, keep up the fab job of holding the incumbent to account.”
Andrew Boff, a veteran Assembly Member who recently launched his sixth bid to be the Conservatives’ mayoral candidate, meanwhile told Mr Rogers: “I regret you pulling out of the race as we need your voice heard in the debate.
“You are bubbling with ideas to improve the lives of Londoners, an excellent advocate for your residents and have an unrivalled experience of transport in the capital.”
