Keith began her public involvement in the feminist movement in high school, where she was the founding Editor of Vanessa and Iris: A Journal for Young Feminists (1983—85), at Brookline High School, Massachusetts. During this same period she also volunteered with WAVAW (Women Against Violence Against Women) in Cambridge, MA where she participated in educational events and protest campaigns. In 1984 she was a founding member of Minor Disturbance, a protest group against militarism from a feminist perspective. In 1986 she was a founding member of Feminists Against Pornography in Northampton, MA. She is a founding editor of Rain and Thunder, a radical feminist journal in Northampton, MA.

As an activist on feminism and more recently ecological issues, Keith has had many appearances, interviews and speeches around the United States and Canada.

Keith was an early public advocate of the American local food movement. In a 2006 Boston Globe human interest story, she said ``I like knowing that I'm supporting the local economy and not corporate America," In The Vegetarian Myth (2009), she sees agriculture as destroying entire ecosystems, such as the American prairie. Agriculture also destroys topsoil. Keith includes “slavery, imperialism, militarism, class divisions, chronic hunger, and disease” as historical outcomes of over-dependence on mass cultivation.

Keith's most recent book is Food, Justice, and Sustainability (2009), an examination of the ecological effects of agriculture and vegetarianism. Pulitzer Prize winning author Alice Walker endorsed it saying "[The Vegetarian Myth] is one of the most important books people, masses of them, can read, as we try with all our might, intelligence, skill, hope, dream, and memory, to turn the disastrous course the planet is on."

Keith is associated with the Deep Green Resistance movement. Her views have attracted negative attention from some vegetarians, what one journalist has called a "Vegan War". Vegan dietician Ginny Messina blogged that Keith's latest effort is "page after page of contradictions, fabrications, and misinterpretations," and that it "is ultimately a sad book."

Keith is currently working on a book with Aric McBay and Derrick Jensen about "resistance strategy" as part of the Deep Green Resistance movement.