Karen Mills, administrator the U.S. Small Business Administration, will leave the agency at the end of next month. (Joshua Roberts/BLOOMBERG)

The Small Business Administration’s administrator will step down next month, the agency’s spokeswoman said Thursday, potentially leaving the agency leaderless for an unknown period of time.

SBA Administrator Karen Mills announced her resignation in February but had pledged to stay in place until a successor was confirmed. But the Obama administration has yet to name a replacement, and Mills told her staff Wednesday evening that she will depart at the end of next month.

“The agency is working closely with the White House on transition plans and on the appointment of a successor,” Emily Cain, the SBA’s spokeswoman, said in an e-mail.

Mills has said she would be joining both Harvard Business School and Harvard’s Kennedy School of Government.

White House officials did not return calls seeking comment on when a successor for Mills would be announced, but the confirmation process typically takes months.

The SBA vacancy is the only open seat on President Obama’s second-term Cabinet for which the administration has yet to put forth a nomination.

The delay in nominating a new administrator is “very disappointing,” said House Small Business Committee Chairman Sam Graves (R-Mo.). The Obama administration should “find a new administrator as quickly as possible,” he said.

Graves warned that the timing could pose problems for the White House, noting that lawmakers are preparing for a lengthy summer recess and that former SBA deputy administrator Marie Johns, the agency’s second-ranking official, departed this year.

“With the deputy administrator also vacant, I’m concerned that a confirmation process could leave the Small Business Administration without a leader well into the fall,” Graves said, adding that the agency “performs an important role for many small businesses.”

Mills was appointed to her current role in April 2009. Obama elevated her position to Cabinet-level status in January of last year.

The SBA backed more than $106 billion in loans to small companies during her tenure, which included record years of more than $30 billion worth of loan guarantees in both 2011 and 2012. Mills led a charge to make it easier for small businesses to apply for various government-backed loans, launching online portals for disaster-loan applications.

But the SBA under Mills’s leadership has taken heat for the federal government’s continued failure to meet its small-business contracting goals in each of the past four years and for slow delivery of disaster aid after Hurricane Sandy. Meanwhile, some have criticized Mills for shifting the agency’s focus away from micro-businesses to support midsize companies.

In the agency’s latest budget proposal, for instance, officials proposed cutting funding for popular small-business counseling programs, angering some members of Congress.

“You have been focused a lot on the medium-sized small businesses, if you will, and we have had previous testimony that there are a lot of jobs created there,” Rep. Kurt Schrader (D-Ore.) said to Mills during a hearing about the budget in May. “But there are many of us who are really worried, with the recession, about a lot of new people starting their own very small business.”

Mills plans to assume a position with the Kennedy School’s Institute of Politics on Sept. 1. On Jan. 1, she will join the business school as a senior fellow, working on the school’s multi-year U.S. Competitiveness Project, said Jim Aisner, director of media relations at the business school. Mills received an MBA from Harvard in 1977.