Regan graduated with a degree in English from Harvard University in 1940. He then attended Harvard Law School, dropping out to join the Marine Corps at the start of World War II. After the war, he joined Merrill Lynch, rising to the position of CEO.
As Chief of Staff, Regan was noted for his disputes with First Lady Nancy Reagan, as well as President Reagan's other advisors, due to his tight control over the President's schedule, and their fear that he exercised undue influence over the President. After the Iran-Contra scandal, in 1987, Regan resigned. Many accused Mrs. Reagan of being responsible for his removal, but the First Lady denied the charge, stating in her memoirs My Turn that there was never "a plot to get rid of Don Regan. There was no cabal. I wasn't in cahoots with anybody to bring about his downfall."