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.

"She brings unique insight in public policy, international diplomacy and economic development that will add further dimension and breadth to our already well-regarded Board of Directors," Fink said in a statement. "Throughout her career, she has held important positions in business and has proven her capabilities in a rapidly expanding and changing marketplace. She will make an exceptional addition to our Board."

Mills has spent a career in and out of politics, as an administrator and political and policy adviser as well as a lawyer. She was deputy counsel in President Bill Clinton's White House and defended him during the 1999 impeachment trial.

In the early 2000s, Mills was a top administrator at New York University, but returned to politics as a senior adviser on Hillary Clinton's 2008 presidential campaign. She ran Clinton's transition to the State Department, closely overseeing the secretary's office there. Mills was particularly instrumental in managing relief and development efforts in Haiti following the deadly earthquake of 2010.