Howard A. Kahn, the chief executive of L.A. Care Health Plan, said he plans to leave in January after leading the large Medicaid managed-care provider for 13 years.
The organization announced the move late Friday. Kahn said it was time to do something different.
During his tenure, L.A. Care grew from serving 700,000 people to 1.6 million, making it a national leader among Medicaid plans.
In October, L.A. Care expanded into the broader insurance market by selling policies through California's health exchange as part of the Affordable Care Act.
"It was clear that our board preferred that I stay through the end of 2014," Kahn said. "This has been the greatest work one could hope for."
Dr. Thomas Horowitz, chairman of L.A. Care's board of governors, praised Kahn's leadership during challenging times for the healthcare industry.
"We are lucky to have had someone of his caliber steering this ship through such turbulent waters," Horowitz said in a statement. "Over the past several years, L.A. Care has been asked to take on an increasing number of fragile and medically needy individuals."
L.A. Care didn't give any details on the search for a new CEO.
In the state exchange, L.A. Care has posted modest numbers in the first three months of enrollment. Through Dec. 31, it signed up 8,053 people in L.A. County, for a 6% market share, in the the only region where it is participating.