ABC's Mike Levine ( @MLevineReports) reports:
"For more than four years I have had the privilege of serving President Obama and his Administration as the Secretary of Homeland Security," Napolitano said in a statement on Friday. "The opportunity to work with the dedicated men and women of the Department of Homeland Security, who serve on the frontlines of our nation's efforts to protect our communities and families from harm, has been the highlight of my professional career."
Napolitano, who has been serving in her current post since 2009, was the first woman to helm of the Department of Homeland Security. A Democrat, she previously served as governor of Arizona from 2003 to 2009 and before that was the state's attorney general.
Napolitano notified President Obama of her decision to step down a month ago, sources say. She is planning to leave her post in early September.
"Since day one, Janet has led my administration's effort to secure our borders, deploying a historic number of resources, while also taking steps to make our immigration system fairer and more consistent with our values," President Obama said in statement. "And the American people are safer and more secure thanks to Janet's leadership in protecting our homeland against terrorist attacks."
Her departure paves the way for what could be a rocky confirmation battle for whomever President Obama decides to appoint to succeed her at a time when the administration's nominees are facing delays in Congress.
Current U.S. Citizenship and Immigration Services department director Alejandro Mayorkas has been nominated to be Deputy Secretary for DHS. His confirmation hearing is July 30. If he is confirmed before Napolitano's departure, he will become acting secretary. However, if he is not confirmed before then, Homeland Security Undersecretary Rand Beers will become the senior leader of DHS, but due to law will not have "acting secretary" title.
Napolitano is preparing to take over leadership of the 10-campus University of California system - the the state's marquee public higher education network that has been plagued by budget cutbacks in recent years. A U.C. spokesman said that her appointment would be made official at a July 18 meeting of the system's Board of Regents in San Francisco that Napolitano plans to attend.
"I will be nominated as the next President of the University of California to play a role in educating our nation's next generation of leaders," Napolitano said. "I thank President Obama for the chance to serve our nation during this important chapter in our history, and I know the Department of Homeland Security will continue to perform its important duties with the honor and focus that the American public expects."
ABC's Ann Compton contributed reporting.