The agency picked up the business after a pitch against M&C Saatchi and will now be responsible for Grant Thornton's advertising activity in more than 100 markets.
A brand campaign is expected to roll out later this year for the firm, which employs more than 30,000 staff worldwide.
Jonathan Geldart, the company's global director of marketing communications, kicked off a review of its advertising in January.
Tangible UK is among the agencies that Grant Thornton has worked with in the past. The agency created a global outdoor, press and digital campaign for the brand.
In the past year, a number of big accountancy firms have made changes to their ad arrangements, including KPMG and Ernst & Young, which appointed JWT and The Assembly respectively.