She and Lorraine Rothman were leaders of a group that founded the Feminist Women's Health Center in Los Angeles, California, in 1971. Downer and Rothman promoted group meetings where the women did self-administered cervical exams and promoted a procedure called menstrual extraction in which the group suctioned out a member's menstrual material out on or about the time of the menstrual period; if she were pregnant, this would constitute a non-professional abortion. In 1973, after the Roe v. Wade Supreme Court decision, their group established women-controlled clinics in Los Angeles and Orange County. Over the next two years, other Feminist Women's Health Centers were established which became part of the Federation of Feminist Women's Health Center in 1975.
Downer began her reproductive rights career on the Abortion Task Force of NOW with Lana Clarke 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. Downer and Rothman travelled across the country and many Self-Help Clinics were formed. In 1972, Downer was arrested and charged with "practicing medicine without a license" – for using yogurt to treat a woman's vaginalyeast infection. She was acquitted after a trial that was dubbed "The Great Yogurt Conspiracy". In 1972 she also gave a notable speech to the American Psychological Association on September 5th, 1972, in Hawaii, entitled "Covert Sex Discrimination Against Women as Medical Patients." 
From 1987 to 1991, Downer attended law school and worked for the Federation of FWHCs. Since then, she has practiced law, mostly in the area of disability rights. In 1981, she was the general editor of A New View of a Woman's Body, published by Simon and Schuster, and she was an editor of a companion book, How to Stay Out of the Gynecologist's Office, published by Women to Women Publication. In 1984, she and Francie Hornstein assisted Ginny Casside-Brinn, R.N. in writing Woman-Centerd Pregnancy and Birth, published by Cleis Press. 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 of the National Abortion Federation.