London Transport Commissioner Mike Brown is to leave Transport for London (TfL) to head up the restoration of the Houses of Parliament.
TfL said Mr Brown will continue in his role, which he took up in July 2015, for the remainder of Sadiq Khan's current term as mayor, which expires next summer.
Mr Brown with the Houses of Parliament in the background.
He will become chair of the Delivery Authority for the Restoration & Renewal of the Houses of Parliament.
He said: 'Every day, thousands of unsung women and men go to work to keep London moving and to make this city better, cleaner and safer for everyone. I am inspired by them and I will miss them, but it's the right time for me to move on to a new challenge after more than 30 years in the transport industry and what will have been nearly five years as Commissioner.
'I'm proud of what has been achieved at TfL and I'm proud of having served and delivered for the mayor. In spite of the challenges that London has experienced at times over the years, TfL is an organisation full of the energy, ability and passion to make this an even better city for all.'
Mr Khan, said: 'Mike deserves huge thanks for the exceptional service he has given to London, both for the excellent job he has done as TfL Commissioner, but also for all the work he has done for the capital since starting at TfL in 1989.
'The recent transformation of the London's public transport network into one of the very best in the world is testament to Mike's expertise, and much of the exciting investment we're continuing to see in transport across the capital is a result of Mike's leadership and vision.’
Mr Brown joined London Underground (LU) in 1989 and became chief operating officer in 2003. He left to run Heathrow Airport but returned as the managing director of LU.
TfL said he has transformed the efficiency of the organisation and has overseen a step-change in its focus on tackling safety on London's roads, improving its air quality and encouraging a shift to public transport, cycling and walking.
It added that he improved reliability across the Underground, oversaw huge station redevelopment projects and led the successful delivery of the services to support the Olympic and Paralympic Games in 2012.