Joanne McCartney is a British barrister and Labour Party and Co-operative Party politician. Since 2004, she has served as a member of the London Assembly, representing Enfield & Haringey. Following the election of Sadiq Khan as Mayor of London in 2016, McCartney has served as Statutory Deputy Mayor of London.

The London Assembly is a 25-member elected body, part of the Greater London Authority, that scrutinises the activities of the Mayor of London and has the power, with a two-thirds supermajority, to amend the Mayor's annual budget and to reject the Mayor's draft statutory strategies. The London Assembly was established in 2000. It is also able to investigate other issues of importance to Londoners, publish its findings and recommendations, and make proposals to the Mayor.