Professor Williams teaches and writes in the areas of criminal law and procedure and evidence. He has been a lawyer for many high-profile clients, from Dick Cavett to Mumia Abu Jamal to the government of Cuba. His litigation victories have made an indelible mark on the law. He was lead counsel in the landmark case of New Jersey v. Michaels, a ruling that led to widespread changes in the way child-abuse investigators interview children. For his work in this area, he was awarded the Morton Stavis Memorial Justice Award. He recently won a death-penalty appeal that led to the dismantling of New York's capital punishment statute. For his death penalty work, he received the Thurgood Marshall Award.

Professor Williams has been active in human rights work in Central America and has spoken on criminal justice issues in the United States and Europe. Before coming to Northeastern, he was a deputy capital defender at the New York Capital Defender Office. He has written about legal theory, contributed entries to the Encyclopedia of Crime and Punishment and other books, and has published articles on trial practice and evidence in many venues. He is the author of Executing Justice: An Inside Account of the Case of Mumia Abu Jamal (St. Martin's Press), a text that one critic called "one of the most important books on race in America today." He also teaches trial practice and death-penalty defense workshops at the Trial Lawyers College in Wyoming.