When state officials and government regulators were concerned that some counties in the United States would be left without a carrier willing to sell insurance under the Affordable Care Act, Mr. Neidorff agreed to provide coverage in some of those markets, said Jesse Hunter, a former chief strategy officer for Centene, and “there was never any hesitation.”

“We want to help out where we are able to,” Mr. Neidorff told The New York Times in 2017.

He was born in 1942 in Altoona, Pa., to A. Harvey Neidorff, a physician, and Shirley Rubin Neidorff, a nurse. He had considered going into medicine but decided instead to major in political science at Trinity University in San Antonio. He later earned a master’s degree from St. Francis College in Brooklyn.

He is survived by his wife, Noémi; his brother, Robert; his sister, Susan Neidorff Reinglass; and his son, Peter. His daughter, Monica Neidorff, died in 2021.

Mr. Neidorff had worked for a unit of UnitedHealthcare when he was recruited to be chief executive of Coordinated Care, the small managed-care company that would become Centene. “It was obvious they needed a complete change,” said Robert Ditmore, a longtime Centene board member who was involved in the company.

Mr. Neidorff expanded aggressively into the Medicaid market, despite concerns about whether it would be a successful venture. “I had questions about Medicaid; is it going to work?” Mr. Ditmore said. But, he added, Mr. Neidorff persuaded him: “It was obviously a big market, so we said let’s go for it.”

Mr. Neidorff took the company public in 2001. In 2016, he acquired a California insurer, Health Net, which offered private plans for another government program, Medicare.

When the Affordable Care Act provided an opportunity to sell low-cost private insurance through the state markets set up by the federal government, Mr. Ditmore recalled, Mr. Neidorff was eager to become a major player, despite uncertainty about how insurers should price their policies in the new market. “He kept pushing for it,” Mr. Ditmore said. “He was a bulldog with a bone in his mouth.”