By Deborah Solomon 

Mary Miller, a top U.S. Treasury Department official who played a key role managing U.S. debt markets and overseeing the financial regulatory overhaul, plans to step down from her post in early September, according to the Treasury Department.

News of Ms. Miller's resignation was announced internally today. She is currently Treasury's Under Secretary for Domestic Finance. The Obama administration hasn't yet decided on a successor.

"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 Jacob Lew said in a statement. "From the debt limit debates and the intricacies of debt management to financial reform and housing finance, Mary has been committed to strengthening our country, improving our financial system and lifting obstacles for working Americans."

Since joining Treasury in 2010, Ms. Miller has played a central role in the administration's increasingly frequent negotiations with Congress over whether to raise the so-called debt ceiling, which limits the amount of government borrowing. As the U.S. bumped up against that limit Ms. Miller has had to manage the nation's debt to prevent the U.S. from defaulting on its obligations, including using so-called "extraordinary measures" to shift government funds between accounts so the U.S. could continue paying its bills.

She spearheaded the creation of so-called floating rate securities--Treasury's first new debt product in 15 years--which is partially aimed at offsetting risk associated with a looming wave of government debt coming due. The note are also somewhat of a hedge on the future direction of the economy, allowing the rates on the floating notes to stay low if the economy remains weak, rather than paying a fixed rate.

Ms. Miller is perhaps best known on Wall Street for her regulatory reform role. Since the Dodd-Frank Act was signed into law nearly four years ago, she has tried to coordinate the sometimes fractious agencies tasked with writing hundreds of rules. She spent months pushing regulators to reach agreement on the Volcker rule, which bans banks from making risky bets with their own money and took more than three years to finalize. More recently, Ms. Miller has been pushing six federal agencies to finish writing mortgage standards intended to boost the quality of loans. That rule is expected to be finalized in the coming months.

Her work hasn't come without controversy: Ms. Miller has a played a large role in establishing the Dodd Frank-created Financial Stability Oversight Council, which must determine whether any large, nonfinancial firms should be considered for designation as "systemically important" and drawn in for Federal Reserve oversight. The FSOC's move to consider whether large asset managers such as BlackRock Inc. and Fidelity Investments pose systemic risk has been blasted by the industry and Ms. Miller has tried to combat the criticism by assuring markets no decision has yet been made.

"It's a bit of an overreaction [to] certainly the public statements that we've made," she said at a government-led conference on asset managers last month.

Ms. Miller, who has been commuting to Washington from her home in Baltimore for 4 1/2 years, didn't intend to stay through the end of the Obama administration and wanted to give Mr. Lew--and President Barack Obama--the opportunity to pick a successor who could have at least two years in the job, according to people familiar with her thinking.

Before coming to Treasury Ms. Miller, 53, spent 26 years at T. Rowe Price Group, Inc., where she headed the fixed income division and sat on the firm's management committee.

Write to Deborah Solomon at deborah.solomon@wsj.com

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