Mr Darling, who served as chancellor from 2007 to 2010, will to take up the role from January 1 next year.
The news of his appointment follows the announcement that Gordon Brown - who served as prime minister when Mr Darling was chancellor - had joined a newly-established advisory panel at Pimco, marking his first corporate role since quitting politics in 2014.
James Gorman, chairman and chief executive at Morgan Stanley, said: “As chancellor of the exchequer Mr Darling played a central role in responding to the financial crisis. The board, our management and our shareholders will greatly benefit from his experience and perspective.”