W was awarded the work following an AAR-led pitch process, with the brief expanding on its previous corporate focus to include consumer media relations, reputation and issues management, b2b and social media.
The agency will also handle both national and international work for the company, which last year had a majority stake snapped up by US-based equity firm The Carlyle Group for an estimated £300m.
With a fleet of more than 3,500 taxis across London, the company has a strong presence in a sector that is estimated to be worth £3bn a year. It recently reported that turnover had risen 7.6 per cent to £171.7m for the year ending August 2013.
However, it is facing stiff competition from start-ups such as Hailo and Uber, which last week gained a publicity boost after protests by black-cab drivers brought parts of central London to a standstill.
W founder Warren Johnson pointed to an "exciting time in the taxi business" with a growing market and "lots of new competition moving in".
"We've been brought in to help Addison Lee with its next stage of growth to help continue its trajectory, while putting its technology offering and ease of booking at the heart of the story," he added.