University authorities announced the change of heart over pay after a meeting with campaigners on Tuesday.
Academics, students and community groups had protested at a refusal to pay cleaners £7.85 per hour.
There are now nine London universities promising to pay the living wage.
Living wage campaigners describe the decision as a "significant victory".
The Citizens UK community group and University of London Union have been calling on all London universities to pay the living wage of £7.85 per hour - which is claimed to be the lowest rate needed to live in the capital.
This is higher than the national minimum wage, which will be £5.93 per hour from October.
UCL had initially rejected calls for contract staff, such as cleaners and catering staff, to receive this level of pay.
But the university came under sustained pressure from students, staff and Citizens UK.
Campaigners contrasted the difficulties of low-paid cleaners, who might have to work several jobs to keep their families, with the £404,742 annual pay package of UCL provost, Professor Malcolm Grant.
Academics at UCL had written to complain about the "grossly inequitable" difference in pay between the highest and lowest paid at the university.
Professor Grant has already announced that he would take a 10% pay cut, as well as a pay freeze.
The university has now agreed to introduce the living wage, including staff working for outsourcing companies, over the next two years
A spokesman for Citizens UK says that Professor Grant "has clearly heard the arguments of the campaign that a living wage is both the right thing to do, and makes good business sense".
UCL is the latest London university to sign up for this higher rate of pay - with commitments already made by Queen Mary, London School of Economics, Birkbeck, Goldsmiths, School of Oriental and African Studies, London Business School, Institute of Education and the London School of Hygiene and Tropical Medicine.
On Tuesday, the newly-elected Labour leader, Ed Miliband, promised his support for the living wage in a speech to his party conference.
London Mayor Boris Johnson has also backed the living wage campaign, arguing that people should be "better off in work than out of work".