WASHINGTON — Health and Human Services Secretary Kathleen Sebelius has resigned, ending a turbulent five-year tenure during when she become the public face of the troubled federal health insurance marketplace, HealthCare.gov.A statement from HHS confirmed her departure: From her work on Head Start to expanding mental health coverage, to advancing cutting-edge healthcare research and, of course, her unwavering leadership in implementing the Affordable Care Act, Secretary Sebelius often calls her work here the most meaningful of her life. As she closes this chapter, Secretary Sebelius is extremely thankful to President Obama and very proud of the historic accomplishments of this administration.President Barack Obama nominated Sylvia Mathews Burwell, the director of the White House Office of Management and Budget, to replace Sebelius in a Rose Garden ceremony Friday morning.Obama praised Sebelius for helping to steer his health care law's comeback after a rocky rollout, even as he nominated a successor aimed at helping the White House move past the political damage."Under Kathleen's leadership, her team at HHS turned the corner, got it fixed, got the job done," Obama said. "And the final score speaks for itself."Obama callec Burwell "a proven manager" who knows how to get results. The nomination of Burwell, who was unanimously confirmed by the Senate for her current post last year, appeared aimed at avoiding an election-year confirmation fight."Last time, she was confirmed unanimously," Obama said. "I'm assuming not much has changed since that time."Sebelius notified the president of her plan to resign in early March.At the time, Sebelius told the president she was confident in the direction the healthcare law implementation had taken after a tough start. After the enrollment period closed, she felt it was time for new leadership at the department.More than 7.5 million people have signed up for marketplace coverage nationwide, Sebelius said Thursday in congressional testimony.In Burwell, Obama will have a nominee with a strong background in management, implementation and performance. At OMB, Burwell has helped navigate the government shutdown in October 2013 and reach a two-year budget agreement with Congress that Obama signed into law.Burwell also served in leadership positions at the Bill & Melinda Gates Foundation, the Walmart Foundation and MetLife. She was confirmed unanimously by the Senate to lead OMB and has a record of working well with both Republicans and Democrats.Sebelius, a former governor of Kansas, was Obamas choice to lead the implementation of the health-care law, the presidents signature legislative accomplishment.Although the fortunes of the Affordable Care Act and the troubled federal insurance marketplace have improved significantly in recent months, Sebelius remained a polarizing figure in the Obama administration.Members of Congress complained that her department was slow to respond to requests for information, and her relationship with the Washington press corps was strained.