Rebekah Mercer, an influential donor to conservative causes — including groups that deny climate science — has quietly stepped down from the American Museum of Natural History, one of the country’s most prominent science museums.

The museum, where Ms. Mercer served two three-year terms, said Thursday through a spokeswoman: “Ms. Mercer’s term expired in December 2019 and her position on the board came to an end at that time.”

Members may serve up to three consecutive terms; the museum did not say if Ms. Mercer had sought election to a third term. Ms. Mercer did not respond to messages seeking comment.

Her presence on the board prompted protests after The New York Times reported in 2017 that the Mercer Family Foundation — which Ms. Mercer operates with her father, the New York investor Robert Mercer — had given nearly $8 million to organizations including those that reject the scientific consensus on climate change. Among these beneficiaries are the Heartland Institute in Illinois and Breitbart News, the influential right-wing website that was run by Stephen K. Bannon, Mr. Trump’s former chief strategist.