He had previously worked for the late civil rights lawyer William Kunstler (Kunstler & Kinoy) and the California Rural Legal Assistance, where he served as Director of Legal Research and Senior Litigation Attorney. After many years as a private practitioner in San Francisco, Kates was an associate with the firms Trutanich & Michel and the Law Offices of Donald Kilmer. He also maintained a civil liberties and rights practice that specializes in the right to bear arms.

In addition, he had been Trustee for the Poverty Lawyers for Effective Advocacy, Member of the California State Advisory Committee to the U.S. Civil Rights Commission, Director of Litigation and Deputy Director for the San Mateo Legal Aid Society, Police Legal Advisor for the San Francisco Sheriff’s Department, Research Fellow at the Pacific Research Institute, and consultant to the legal services program for the cities of Seattle and Berkeley and the state of Alaska. During the recent Supreme Court case, District of Columbia v. Heller, Kates served as an advisor to the council in the Court of Appeals.

His books include Armed: New Perspectives on Gun Control, Restricting Handguns: The Liberal Skeptics Speak Out, Firearms and Violence: Issues of Public Policy, and The Great American Gun Debate: Essays on Firearms and Violence (with Gary Kleck). He was also contributed to numerous scholarly journals, such as the Harvard Journal of Law & Public Policy, American Journal of Criminal Law, Journal of Law, Economics & Policy, and Constitutional Commentary.