Karen Mills is stepping down as the head of the Small Business Administration after nearly four years on the job.

Mills, 59, announced her resignation in a letter to SBA employees Monday. She intends to stay on until a successor is confirmed.

"I want to take this opportunity to thank everyone across the SBA for the work you have done to help America's entrepreneurs and small-business owners during this critical juncture in our nation's history," Mills said in the letter.

A native New Englander, Mills ran a private-equity advisory firm in Brunswick, Maine, before she was tapped by President Obama to run the agency, which has since been elevated to a Cabinet-level post. Before forming her own advisory firm, Mills worked for General Foods Inc. and the consulting firm McKinsey & Co. She also once served as a top economic development advisor to former Maine Gov. John Baldacci.

Mills spent much of her early tenure championing the Obama administration's small-business policies, including the 2010 Small Business Jobs Act that contained a number of incentives aimed at boosting small-business lending.

The SBA's primary mission is to provide government guarantees to lenders, and on her watch the SBA supported $106 billion of loans to small businesses, including a record $30.5 billion in fiscal year 2011. Incentives, such as higher guarantee levels and the temporary elimination of guarantee fees, helped boost overall demand.

Last fiscal year the agency supported $30.3 billion in small-business loans, second most in its 60-year history, without the aid of incentives, which expired at the start of 2011.

The SBA also makes direct loans to families and businesses harmed by natural disasters, and under Mills the agency streamlined the disaster loan application process, resulting in faster turnaround times.

In her letter, Mills noted that after Hurricane Katrina applicants waited an average of 77 days to receive SBA loans. Following Hurricane Sandy last year, more than half of requested loans were turned around within three weeks.

"Four years ago, when I arrived at the SBA, America's small businesses and entrepreneurs were struggling in the face of the worst economic environment since the Great Depression - and a banking sector that was frozen," Mills wrote in the letter.

"Together, we rolled up our sleeves and went to work. And from day one, each of you stepped up and fulfilled the mission of what the agency was created to do."

However, the SBA's momentum could be slowed unless Congress and the White House can reach an agreement on how to avert automatic cuts in federal spending due to kick in March 1. The White House said last week that the automatic cuts - known as sequestration - would reduce government guarantees on small-business loans by nearly $900 million.

Mills is expected to return to Maine full time after she leaves the SBA. The agency did not say if she plans to return to the private sector or pursue other opportunities.

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