WASHINGTON — Karen Mills announced Monday that she plans to step down as leader of the U.S. Small Business Administration, a position she held since April 2009.

Mills, the wife of Bowdoin College President Barry Mills, lives in Brunswick. After a career as a venture capitalist, she won unanimous U.S. Senate approval to serve as the 23rd leader of the U.S. Small Business Administration.

President Barack Obama elevated the administrator of the Small Business Administration to a Cabinet level position in January 2012. Mills, a Democrat, is the only person to serve in that role during his presidency and is the only Maine resident to serve in Obama’s Cabinet. She joins a number of Obama’s other first-term appointees who are leaving the administration during the early weeks of the president’s second term.

“Over the last four years, Karen has made it easier for small businesses to interact with the federal government by reducing paperwork and cutting through red tape,” Obama said in a release Monday morning. “She has played a leading role in my administration’s efforts to support start-ups and entrepreneurs. And she was instrumental in the passage of the Small Business Jobs Act. Because of Karen’s hard work and dedication, our small businesses are better positioned to create jobs and our entire economy is stronger.”

Sen. Susan Collins, who served as New England regional director for the SBA in 1992, said in a statement that Mills called her Monday morning to inform the senator that she intends to leave the SBA position as soon as a successor is confirmed.

“Even during these tough economic times for our nation’s small businesses, Karen brought drive, determination and dedication to her position at SBA,” said Collins, a Republican. “She clearly understood that small businesses are our nation’s job creators and they are the backbone of our economy.”

In a letter Monday to SBA staff, Mills touted the agency’s role in helping small businesses through “the worst economic environment since the Great Depression — and a banking sector that was frozen.”

Among the SBA’s accomplishments during her tenure, Mills listed facilitating more than $106 billion in loans to at least 193,000 small businesses, revamping the agency’s disaster response program, streamlining application processes and making it easier for small businesses to land federal contracts.

“Small businesses are now ready to go on the offensive,” she wrote. “They are ready to expand to new markets, to scale their operations and to hire new workers.

Mills also described the agency as being stronger than it was when she arrived.

In addition to her work in the private sector, most recently as president of the MMP Group, Mills worked closely with former Gov. John Baldacci on a number of Maine state government initiatives before Obama picked her to head the SBA. She played a lead role in rallying voters’ support for a $50 million research and development bond in 2006.

In 2007, Baldacci appointed her to chair the Maine Council on Competitiveness and the Economy, which focused on regional economic development. She also served on the Governor’s Council for the Redevelopment of Brunswick Naval Air Station, which a federal Base Realignment and Closure commission voted in 2005 to close.

In 2008, Mills, a Harvard Business School graduate, co-wrote a Brookings Institution report on regional industry clusters.

Mills, who moved to Brunswick in 2001, told the Bangor Daily News in a January 2012 interview that her experiences working to promote competitiveness, research and development in Maine “are really the foundation stones for many of the things that we’re doing across the country today.”

Other members of Maine’s congressional delegation joined Collins in praising Mills.

Sen. Angus King, I-Maine, who also lives in Brunswick, called Mills a “tireless champion and a vigorous advocate” for entrepreneurs and small-business owners. He cited streamlining application processes and increased resources for startup businesses during the recession as key accomplishments during her tenure at the SBA.

Undoubtedly, her work has been invaluable in strengthening the backbone of our economy — our small businesses — and I both commend and thank her for it,” King said.

“Karen has always been an advocate for Maine businesses, and I know that commitment will continue,” said Rep. Mike Michaud, a Democrat who represents Maine’s 2nd District. “I’d like to thank Karen for her service to Maine and the nation, and I wish her well on her future endeavors.”

“It has been great to have someone with Maine sensibilities looking out for our small businesses,” said Rep. Chellie Pingree, a Democrat who represents Maine’s 1st District. “I spoke with her this morning, and I know she is looking forward to coming back to Maine and finding new ways to contribute.”

David Clough, Maine state director for the National Federation of Independent Business, said that Mills’ departure, coming so soon after the retirement of Sen. Olympia Snowe, R-Maine, will remove two influential small-business advocates with Maine connections from economic decision-making in Washington.

An analysis of economic trends that the NFIB released last month shows a higher “optimism index” among small-business owners than when Mills took office, but one still described in the report as “a recession level reading.”

A Washington Post blog reports that Mills’ top assistant, Marie Johns, emailed co-workers Thursday to inform them that she’s leaving the SBA in May.

Last month, Maurice “Moe” Dube, the longtime director of Maine’s U.S. Small Business Administration district office announced his plans to retire.

The SBA employs more than 3,000 people and manages a portfolio of more than $90 billion in loan guarantees.