UNITY — Mitchell Thomashow is stepping down as president of Unity College.

College officials said in a statement today that Thomashow, 61, had turned in his resignation to the college’s Board of Trustees and it had been accepted. The college did not immediately explain why Thomashow was leaving.

Thomashow came to Unity College in 2006 after more than 30 years at Antioch New England Graduate School in Keene, N.H., where he was chairman of the environmental studies department and an associate dean for institutional advancement.

“I want to thank President Thomashow for almost five years of exemplary achievement and tireless effort,” Board Chairman Tim Glidden said in a statement. “Unity College has grown in ways that would have been hard to imagine in 2006. Our national stature has never been higher. Our faculty is engaged in a transformative overhaul of the college’s programs. The face of the campus itself is changing as we upgrade our facilities. The students are deeply engaged in the biggest issues that face our planet.”

College officials said they would begin a national search for the college’s 10th president, hoping to hire someone by mid-summer.