When did the former U.S. Secretary of Homeland Security assume the office of White House Chief of Staff?
Elaine Duke:  She became acting Secretary of Homeland Security on July 31, 2017, when John F. Kelly assumed the office of White House Chief of Staff. John F. Kelly: John Francis Kelly (born May 11, 1950) is the current White House Chief of Staff for U.S. President Donald Trump, previously serving as U.S. Secretary of Homeland Security in the same administration.
July 31, 2017