(Adds background about her role)
WASHINGTON, June 12 (Reuters) - Mary Miller, the U.S. Treasury's top official for domestic finance and a key player in implementing the Dodd-Frank Wall Street reforms, will step down in early September, Treasury said on Thursday.
As Treasury undersecretary for domestic finance, Miller was also effectively custodian of the country's public debt, operating largely behind the scenes in Washington power battles over the debt ceiling, the statutory limit on the nation's debt.
"For over four years, Mary has worked tirelessly on behalf of the American people, leading our department's efforts to tackle some of the most difficult challenges facing our country," Treasury Secretary Jack Lew said in a statement.
"I know that our country is far better off for all that she has accomplished," he said.
During political fights in Washington over whether to raise the debt limit, Miller traveled around the world to reassure foreign debt holders the United States would not default on its obligations. She also tried to predict what might happen if Congress failed to raise the debt ceiling.
On the financial regulatory front, Miller was one of the leading players in setting up the Financial Stability Oversight Council, a panel of regulators created by the 2010 Dodd-Frank law tasked with policing the market for emerging risks. The panel has the power to designate large financial firms as "systemic" - a tag that subjects them to tougher regulations.
In that role, she was involved in designating large firms including GE Capital, AIG and Prudential. Last month, Miller helped organize and run a public meeting to explore if the activities of large asset managers may pose market risks, which the industry heavily protested.
Miller became undersecretary in March 2012 after serving as assistant secretary for financial markets, responsible for managing public debt. Miller also spent 26 years at the investment firm T. Rowe Price Group Inc, where she was a member of the management committee.
A source familiar with Miller's thinking said she has not yet decided her future plans. But she wanted to announce her departure now to give the administration plenty of time to fill her role, said the source, who spoke on condition of anonymity. (Reporting by Timothy Ahmann and Sarah Lynch, additional reporting by Anna Yukhananov; Editing by James Dalgleish, Meredith Mazzilli and Chizu Nomiyama)