City Hall said the highly-regarded former Labour MP, who joined Mr Khan’s top team in May 2018, was taking a short break to consider her next career move.
She has worked non-stop since the start of the pandemic trying to keep Tube and bus services running, while also having to contend with numerous other issues, from the delays to the opening of Crossrail, the Hammersmith bridge fiasco and several exhausting rounds of Government bail-out negotiations.
Sources said she had “done her tour of duty” and denied rumours of tensions between City Hall and TfL, where some executive posts are understood to being reconsidered by TfL commissioner Andy Byford.
Ms Alexander, who also became a role model as she took up cycling after overcoming her fears of riding in London, will be replaced by former Labour MEP Seb Dance.
City Hall said she had planned to stand down in May, at the end of Mr Khan’s first term – which was extended by a year due to the pandemic – but decided to stay on to help secure the latest temporary TfL bailout, which runs until the start of February.
Ms Alexander’s decision could also pave the way for a return to front-line politics. She was formerly the shadow health secretary but quit the role in 2016, prior to stepping down as MP for Lewisham East to join Mr Khan at City Hall, in protest at Jeremy Corbyn’s leadership of the Labour party.
She will remain on the TfL board until Crossrail opens. This is due to happen between February and June next year.
She said: “It has been an enormous privilege to work with Sadiq and so many brilliant colleagues at both TfL and City Hall during the last three-and-a-half years.
“I have learnt from some exceptionally talented people and I am tremendously proud of the way we kept London moving through one of the most difficult periods in its history.
“The work we have done on Crossrail, protecting free travel for children and young people, cleaning up London’s air and making cycling safer is critically important for our city and I am pleased to have been able to make a positive contribution to those efforts.”
Mr Khan said: “I want to thank Heidi for her exceptional service to London as deputy mayor for transport. Heidi has worked tirelessly to support our public transport during the last three-and-a-half years.
“In particular, I’d like to pay tribute to Heidi for tirelessly leading TfL through the pandemic – the most difficult period in its history.
“She is without doubt one of the most dedicated, resilient and respected politicians I have ever worked with and I wish her all the best for the future.
“I am delighted that Seb Dance has agreed to become our new deputy mayor for Transport. I look forward to working with him to strengthen our public transport network, ensure the Elizabeth Line is a success and continuing to improve TfL as an organisation.”
Mr Dance was an MEP until the UK withdrew from the European Parliament in 2020 as part of Brexit.
He was vice chair of the environment committee. He has also been a special adviser to Shaun Woodward, the former Tory MP who became the Labour Secretary of State for Northern Ireland, and previously worked for the charity ActionAid UK.
He will start on January 4, and is also due to replace Ms Alexander as deputy chair of TfL. He said: “I am honoured to be joining Sadiq and the team at City Hall at this crucial time for London.
“A high-quality and affordable public transport network is fundamental to London and the UK’s recovery from the pandemic – and I look forward to continuing Heidi’s fantastic work.
“London is without doubt the greatest city in the world – and I am looking forward to working with TfL to deliver a world-leading transport system.”