Karen Mills, head of the Small Business Administration, is stepping down, the White House announced Monday.

The SBA administrator thanked her staff in a statement, concluding a tenure that started with her appointment by President Obama in April 2009. The president elevated her position to Cabinet-level status in January 2012.

Mills’s departure follows that of the agency’s deputy administrator, Marie Johns, who announced her resignation Thursday.

During Mills’s tenure, the SBA backed more than $106 billion in loans to small companies, including record years of more than $30 billion worth of guarantees in 2011 and 2012. Mills also worked to streamline applications for agency resources, launching online portals for federal contracts and disaster-loan applications.

“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 said in a letter to the staff at the SBA. “Together, we rolled up our sleeves and went to work.”

Mills tried to broaden the appeal of entre­pre­neur­ship,targeting women, minorities and senior citizens — the latter of which she began referring to as “encore entrepreneurs.”

“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 Monday, adding that “because of Karen’s hard work and dedication, our small businesses are better positioned to create jobs and our entire economy is stronger.”

The last change under Mills’s watch came just last week, when the agency announced it would increase its available backing for surety bonds, lifting the maximum guarantee from $2 million to $6.5 million for public and private contracts.

But the agency also faced difficulties during her tenure. The federal government missed its stated small-business contracting goals each year, and this past summer, the SBA was criticized for slow approval and delivery of disaster loans to businesses damaged by Hurricane Sandy.

Before joining the administration, Mills worked in the private sector, most notably as president of the MMP Group, which invested in the consumer products, foods, textiles and industrial sectors.

Mills did not share any details about her plans after leaving but said she will stay in place until a successor is appointed.

Mohana Ravindranath contributed to this report.