Transport for London (TfL) boss Andy Byford will leave the organisation at the end of October.
The organisation said Mr Byford – who became transport commissioner in June 2020 – will “resume life in the United States” and is “closing out a 33-year public service career”.
TfL’s chief operating officer Andy Lord will take on the role of commissioner on an interim basis.
The two main priorities Mr Byford set out at the start of his period in charge – opening the Elizabeth line without more delays and leading TfL out of the financial crisis caused by the coronavirus pandemic – have been achieved, according to the transport body.
“I have been blessed to have had such an amazing transport and public service journey, and now is the right time to bow out and resume life in the US with my wife.”
Mr Byford described the opening of the Elizabeth line in May as “without doubt the highlight of my career”.
It was “made truly poignant” by being one of the last major events attend by the Queen, he said.
“It was an honour to meet her, and an honour to oversee the transport arrangements for the ceremonial events to mark her passing.”
Mr Byford helped TfL secure a £1.2 billion long-term funding package from the Department for Transport (DfT) in August.
He said this means he can “leave with TfL set fair to move positively into the future”.
The agreement was only reached after months of tough negotiations between TfL and the DfT.
After the deal was announced, London mayor Sadiq Khan said it was “far from ideal” and warned TfL has a £740 million funding gap in its budget, with fare increases and service cuts under consideration alongside efficiency savings.
Mr Byford said he “hugely appreciated the support of the mayor”, adding that they “navigated some dark days through the toughest period in TfL’s history”.
Mr Khan said the outgoing commissioner “deserves huge thanks for his hard work and dedication”.
He went on: “Andy has provided Londoners with an exceptional service and his work has ensured that despite the challenges we have faced, our public transport network remains world class.”
He added: “I want to thank Andy Byford for his service to Londoners and, as he and his wife move back to the United States, I wish them all the very best”.