The lights at City Hall were extinguished for two hours on Saturday as part of the Blackout London evening to encourage people to take action on climate change.
Green Party members of the London Assembly Darren Johnson and Jenny Jones won the backing of Ken Livingstone to switch off the lights at City Hall.
"Given the need to reduce energy and tackle climate change I am concerned by the number of major buildings that have their lights blazing throughout the night for no good practical reason," says Darren Johnson.
"Many modern buildings, like City Hall, have been designed to have their lights on all night.
"I love City Hall and it's one of the most iconic new buildings in the capital but it's really quite embarrassing that the lights are left on all night every night when the Mayor is trying to persuade us all to save energy.
"I am pleased that for one night, at least, Ken Livingstone has agreed to switch the lights off."
Blackout London was promoted by a coalition of groups in support of the International Day of Climate Change Action and the I Count Climate Change Rally in Trafalgar Square.