Trustees of Reservations, the country’s oldest land trust, has named Katie Theoharides, a former state energy and environmental affairs secretary, as chief executive officer.

Theoharides, who has two decades of experience in environmental leadership and land conservation, fills a vacancy left by the sudden departure of John Judge in October after just eight months in the job.

The Trustees, founded in 1891, is a pivotal player on the state’s environmental and conservation scene. The Boston nonprofit maintains more than 120 properties — including Crane Beach in Ipswich, Appleton Farms in Ipswich and Hamilton, and World’s End in Hingham — that draw more than 2 million visitors annually. Its more recent site acquisitions include Jewell Hill in Ashby, Armstrong-Kelley Park in Osterville, and Becket Quarry in Becket.

In an expansion of its mission to urban locations, the Trustees has been designated as the site developer for Piers Park III in East Boston.

Theoharides, 41, is a respected environmental and climate science expert. She studied evolutionary biology and ecology as an undergraduate at Dartmouth College and earned a master’s in environmental biology at the University of Massachusetts Boston.

“At her core she is an environmentalist who cares for land conservation and for mobilizing people through a shared reverence for nature,’’ Peter B. Coffin, chair of the Trustees’ board of directors, said in a statement released Wednesday.

Theoharides’s appointment marks a homecoming of sorts. In 2010 she joined the Hilltown Land Trust, a Trustees’ affiliate that serves 13 rural communities in the western part of the state. She was the group’s first non-volunteer executive director.

“This opportunity with the Trustees is a once-in-a-lifetime job,’’ Theoharides said in an interview. “This called to my heart and my passion.’’

Theoharides joined the administration of former governor Charlie Baker in 2016 as director of climate and global warming solutions. She was named secretary of energy and environmental affairs in 2019.

In that role, Theoharides led the state’s ambitious efforts on climate change, where she was responsible for a staff of 3,000 and an annual operating and capital budget of $600 million.

Shortly after stepping down in April 2022, Theoharides was hired by the US unit of RWE Renewables, a German multinational, to run its offshore wind development along the East Coast.

During her time with RWE, Theoharides oversaw the expanding US Offshore Wind project development portfolio from the Gulf of Maine to the Gulf of Mexico.

“We were in startup mode,’’ she said. “I learned a lot about building a new team.’’

Judge, her predecessor at the Trustees, departed abruptly, with he and the board citing “differences in their respective visions.’’ He had taken over from Barbara Erickson, who died in early 2021 at 42 from a rare form of appendix cancer.

Theoharides will officially take the helm in July. She said her focus will be building on the Trustees’ historical strengths of conservation and preservation while working to expand access to its sites.

“We are at a moment in time now that calls for us urgently to preserve the outdoors,’’ Theoharides said. “We need to come together around climate-change solutions.’’

Larry Edelman can be reached at larry.edelman@globe.com.