The Assembly won the business after a pitch against Bartle Bogle Hegarty and Engine that was run through the AAR.
It will aim to provide Ernst & Young, which is one of the world's "big four" accountancy firms, alongside Deloitte, KPMG and PricewaterhouseCoopers, with a new global brand strategy and proposition across different territories.
In August, KPMG appointed JWT to handle a UK-specific advertising brief without a pitch. JWT will be charged with creating a local integrated campaign for the brand.
This summer, Ernst & Young announced it would appoint non-executive directors to its global advisory board for the first time amid growing pressure on accountancy firms over governance and the introduction of a new audit code.