Downer began her reproductive rights career on the Abortion Task Force of NOW with Lana Phelan, author of The Abortion Handbook, who became her mentor. Downer and other women observed abortion procedures at Harvey Karman's illegal abortion clinic on Santa Monica Boulevard in West Los Angeles to learn how to perform abortions themselves. They called a meeting on April 7, 1971 to educate women about abortion and their bodies. The result of this first meeting of the Self-Help Clinic was the development of the concept of menstrual extraction and the invention of the Del-Em kit by Lorraine Rothman. Later that year, Downer was arrested and charged with "practicing medicine without a license" – for using yogurt to treat a woman's vaginal yeast infection. She was acquitted after a trial that was dubbed "The Great Yogurt Conspiracy."
From 1987 to 1991, Downer attended law school worked for the Federation of FWHCs. Since then, she has practiced law, mostly in the area of disabilities rights. In 1992, she wrote A Woman's Book of Choices with Rebecca Chalker, published by Seven Stories Press. She has also served on the Board of Directors for the National Abortion Federation.