During his 1000 days as Chancellor, Darling presided over the bailouts of both Lloyds and RBS and has often described how he was given just two to three hours to save the latter.
Morgan Stanley board members were paid $75,000 for their service in 2014, according to company filings.
They also received between $10,000 and $30,000 for working on or leading a board committee and were awarded $250,000 in stock.
Brown's role at Pimco sees him working alongside Ben Bernanke, the former head of the US Federal Reserve, and former president of the European Central Bank Jean-Claude Trichet.
A spokesman told the BBC on Tuesday that Brown will not financially benefit from his work there.