Cheryl Mills (Michael Buckner/Getty Images) Cheryl Mills (Michael Buckner/Getty Images)

Cheryl D. Mills, the longtime confidant and counselor to former secretary of state Hillary Rodham Clinton, is joining the board of directors of BlackRock Inc., a major global investment firm.

Mills, who served as Clinton's chief of staff at the State Department, has been among the inner circle of advisers helping Clinton chart her plans for the future and could figure prominently in Clinton's campaign should she run for president in 2016.

BlackRock announced Tuesday that Mills would join the company's board Nov. 20 as an independent director. Laurence D. Fink, BlackRock's chairman and chief executive, noted that Mills has served at the highest levels of government for nearly two decades.