Former mayor Ken Livingstone referred to City Hall as a 'glass testicle'. The new mayor, Boris Johnson, has referred to it more politely as 'The Onion'. It is the headquarters of the Mayor of London and London Assembly. The design by Norman Foster is unusually bulbous in an attempt to improve energy efficiency by reducing surface area. The original plan was apparently for a giant sphere hanging over the Thames but when the building arrived in 2002 it was more conventionally attached to the ground.

A 500m helix of a walkway goes to the top of the building taking the visitor 10 storeys up to 'London's Living Room' - an exhibition and meeting space with a viewing deck occasionally open to the public. The walkway is designed to represent transparency by allowing views of most of the interior of the building.

Next to City Hall is a sunken amphitheatre called The Scoop, which is used in the summer months for open-air performances.