WASHINGTON — Karen Mills, the only Maine resident serving in President Obama’s Cabinet, announced Monday that she is stepping down as head of the Small Business Administration after nearly four years on the job.

Mills, who lives in Brunswick, has served as administrator of the SBA since April 2009 and led the agency during a tumultuous time as the Obama administration struggled to get the economy back on course. She is the latest member of Obama’s Cabinet – and the fourth woman – to announce her departure as the administration begins a new, four-year term.

“Serving as SBA Administrator, and seeing this agency rise to meet the economic challenges we faced when I arrived four years ago, has been both a privilege and an honor,” Mills wrote in a letter to staff on Monday.

Mills will stay on the job until her successor is confirmed by the Senate. She did not specify her post-Cabinet plans, although she is occasionally mentioned as a potential candidate for political office in Maine.

Obama elevated the SBA administrator job to a Cabinet-level position, making Mills a key part of his economic team. On Monday, the president praised Mills for her time in office.

“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,” the president said in a statement. “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.”

As administrator, Mills is responsible for an agency with more than 3,000 employees that often acts as a bridge between the government, banks and the nation’s small businesses.

The SBA currently manages more than $90 billion in loan guarantees, supporting more than $30 billion in loans to 60,000 businesses in 2011. Throughout the recession, Mills and the SBA played a key role in trying to convince banks to start lending again to businesses and open up their credit lines. The agency is also assists businesses with federal contracting, navigating export requirements and securing disaster relief and federal grants.

Susan Eckerly, senior vice president for policy at the National Federation of Independent Business, did not give Mills a sterling grade largely because she believes the Obama administration’s policies have not helped small businesses grow. Eckerly directed much of her criticism not directly at Mills but at the Obama administration’s controversial health care law, which NFIB strongly opposed and continues to denounce.

“She had to preside over a very difficult time in our economy and within the [scope] of the administration she tried to use the tools that were available to her,” Eckerly said. “But the bottom line for us and the 800-pound gorilla in the room is the health care law. And she wasn’t able to stop that.”

Eckerly acknowledged Mills probably had limited opportunities to significantly alter the health care law but said “she is part of this administration.”

In her letter, Mills thanked SBA staff for their work and called serving as administrator “one of the greatest experiences of my life.”

Prior to joining the SBA, she worked in the private sector as a venture capitalist, most recently as president of the private equity firm MMP Group in Brunswick. Mills is married to Barry Mills, the president of Bowdoin College.

Her departure is Maine’s second loss in as many months of an influential voice in Washington for small businesses. Maine Sen. Olympia Snowe served as the top Republican on the Senate Small Business and Entrepreneurship Committee until her retirement last year after 34 years in Congress. Snowe also urged Obama to elevate the position to a Cabinet-level post.

Members of Maine’s congressional delegation also praised Mills’ term at the SBA.

“Even during these tough economic times for our nation’s small businesses, Karen brought drive, determination, and dedication to her position at SBA,” Sen. Susan Collins, R-Maine, said in a statement. “She clearly understood that small businesses are our nation’s job-creators and they are the backbone of our economy.  I wish her all the best.”

Sen. Angus King, a Brunswick resident who knows Mills personally, called her a “tireless champion” for small businesses.

“Not only has she successfully worked to improve lending and increase capital for startups, but she’s also streamlined application processes and enhanced programs to make them more effective and friendly tools for small businesses,” King said in a statement. “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.”

Mills’ departure also set off a flurry of speculation on Twitter as to whether she might seek elected office in Maine. Her name has occasionally come up as a possible Democratic contender in the 2014 gubernatorial race.