Brown was promoted to treasure back in 2014, and had been thought to be a possible successor to CEO James Gorman.
She'll be replaced at Morgan Stanley by John Ryan who was formerly the chief investment officer at Morgan Stanley Bank.
(What will move markets this quarter and how should investors position themselves ahead of time? Jim Cramer sat down with four of TheStreet's top columnists recently to get their views. Click here to listen to his latest Trading Strategies roundtable with them and read their advice for stocks, bonds, forex and gold.)