CMS Administrator Marilyn Tavenner, a key figure in the launch of HealthCare.gov, is leaving the Obama administration next month. (AP Photo/Molly Riley, File)

Marilyn Tavenner, a key Obama administration health official overseeing the country's largest health insurance programs, announced Friday that she's resigning from her position as administrator of the Centers for Medicare and Medicaid Services next month.

Tavenner, who's been with CMS since 2010, played a major role in the implementation of President Barack Obama's health-care law. Her agency wrote many of the new rules putting the Affordable Care Act in place and oversees the new health insurance exchanges created by the law. Tavenner was at the center of the disastrous rollout of HealthCare.gov in fall 2013, apologizing to Congress for the faulty enrollment Web site's performance.

Tavenner's email to staff didn't explain why she's stepping down. She faced another HealthCare.gov embarrassment in November, when she acknowledged that CMS had mistakenly inflated Obamacare enrollment by about 400,000 last summer because the agency included people who had bought stand-alone dental plans on the health insurance exchanges.

Tavenner, 63, had been acting administrator of CMS since late 2011, and she was confirmed by the Senate, 91-7, in May 2013 — making her the first permanent head of the massive agency in about six years. CMS oversees insurance coverage for about one in three Americans, making it the largest health-care payer in the country and an influential force in the health-care system.

"It goes without saying that Marilyn will be remembered for her leadership in opening the Health Insurance Marketplace," HHS Secretary Sylvia Mathews Burwell wrote in a separate email to staff. "In so doing, she worked day and night so that millions of Americans could finally obtain the security and peace of mind of quality health insurance at a price they could afford. It’s a measure of her tenacity and dedication that after the tough initial rollout of HealthCare.gov, she helped right the ship, bringing aboard a systems integrator and overseeing an overhaul of the website."

CMS principal deputy administrator Andy Slavitt will become acting administrator when Tavenner leaves at the end of February, according to Burwell's email. Slavitt was hired by the administration last summer after he helped salvage HealthCare.gov and other state exchange Web sites as a technology executive with UnitedHealth Group's Optum division.

Though Tavenner was at the center of numerous Obamacare battles, she enjoyed bipartisan support and was thought to have good working relationships on Capitol Hill. The top Republican on the Senate Finance Committee, whose jurisdiction includes Medicare and Medicaid, praised Tavenner in a Friday statement.

“Marilyn has done a great job in a very difficult position under near impossible circumstances," said Sen. Orrin Hatch of Utah. "She has proven herself to be a strong leader and a straight-shooter who brought in much needed private sector sensibility into the agency. I truly appreciate her service and wish her the very best in her next adventure."

Tavenner's tenture was markedly different from that of her predecessor, Don Berwick, who stepped down from a recess appointment in December 2011 amid staunch Republican opposition to his nomination. Berwick, widely respected in the health-care community, was attacked by Republicans for remarks praising Britain's National Health Service.

Tavenner is the latest CMS official to announce her departure from the Obama administration. Cindy Mann, who's lead negotiations with states to expand their Medicaid programs, announced her departure last month. Melanie Bella, who lead the office coordinating care for people dually eligible for Medicare and Medicaid, also announced in December that she's stepping down.