The chief of the US federal hiring office resigned on Friday after the massive attack on the agency's systems that put the personal data of more than 22 million Americans at risk.
The White House said Katherine Archuleta had stepped down as head of the Office of Personnel Management and that OPM was enhancing cyber-security measures, such as limiting the number of "privileged users" of computer data.
Archuleta, facing a chorus of demands from Congress for her ouster, said in a statement she had told President Barack Obama it was "best for me to step aside and allow new leadership to step in".
Beth Cobert, who works in the White House budget office, will become acting OPM director, the White House said.
The departure of Archuleta will not fix OPM's serious cyber-security weaknesses, which Obama administration officials conceded would take months, possibly years, to address.
Republicans in Congress accused the administration of being flat-footed on the growing problem of computer hacks.
The latest hacking revelation at OPM, revealed on Thursday, followed what the OPM called a "separate but related" computer incursion involving the theft of data on 4.2 million current and former federal workers.
Because many of the same people were affected by both hacks, the total comes to about 22.1 million people, or almost 7 percent of the US population, making the incidents among the most damaging cyber security breaches ever.
Social Security numbers and other sensitive data, including possibly compromising secrets gathered for security clearances, were stolen from OPM computers in the sweeping intrusions.
The United States has identified China as the leading suspect, but China's Foreign Ministry has dismissed the accusation as "absurd logic".
Archuleta's departure came a day after Republican House of Representatives speaker John Boehner issued a statement saying that he had "no confidence" in OPM's current leadership.
On Friday, Boehner - citing chronic problems at the Veterans Administration that led to the resignation of Secretary Eric Shinseki last year - said a change in personnel did not always lead to real change and Obama had to "repair" the OPM problems.
Archuleta, a high-ranking official in Obama's 2012 re-election campaign, was appointed to the top OPM job in May 2013 and sworn in the following November.
The computer hacks at OPM, coupled with computer glitches this week that disrupted operations at both the New York Stock Exchange and United Airlines, have raised serious concerns in Congress about the security of major US computer systems.
OPM sued over breach
The National Treasury Employees Union (NTEU) has sued the Office of Personnel Management over the breach, alleging it violated constitutional privacy rights of NTEU members by failing to keep their personnel records safe.
The lawsuit, filed in the US District Court for the Northern District of California, asked the court to declare the OPM's failure to improve cybersecurity to be unconstitutional.
The lawsuit also asked the court to order OPM to pay for lifetime credit monitoring services and identity theft protection for NTEU members, to take steps to heighten its information technology security to prevent future acts, and to no longer collect personal information from NTEU members electronically until those steps are taken.
"We believe that a lawsuit is the best way to force OPM to take immediate steps to safeguard personnel data, prevent such attacks in the future and help our members protect themselves against the fallout," NTEU national president Colleen Kelley said in a statement.
NTEU, the nation’s largest independent federal union, represents 150,000 employees in 31 agencies and departments.