Kunstler served in the U.S. Army during World War II in the Pacific theater, attaining the rank of Major, and received the Bronze Star. He was admitted to the bar in New York in 1948 and began practicing law. He was an associate professor of law at New York Law School (1950-1951).
Kunstler's image was that of a strident radical who defended controversial clients, including Salvador Agron, Lenny Bruce, William Worthy, H. Rap Brown, Stokely Carmichael, American Indian Movement (AIM) leaders, Filiberto Ojeda Rios, Jack Ruby, Abbie Hoffman, Assata Shakur, Angela Davis, Jerry Rubin, Martin Luther King, Lemuel Smith, Siddig Ibrahim Siddig Ali, Ibrahim A. ElGabrowny, Gregory Lee Johnson, Wayne Williams, Larry Davis, Michael X and Gary McGivern. In the Brown case, Kunstler worked with Baton Rouge civil rights attorney Murphy Bell.
He gained national renown for defending the "Chicago Seven" (originally "Chicago Eight") against charges of conspiring to incite riots in Chicago during the 1968 Democratic National Convention. During the trial, he and co-defense attorney Leonard Weinglass were cited for contempt (the convictions were later overturned).
From 1983 until Kunstler's death in 1995, he employed future radio personality Ron Kuby as a junior partner. The two took on controversial civil rights and criminal cases, including cases where they represented Sheikh Omar Abdel-Rahman, head of the Egyptian-based terrorist group Gama'a al-Islamiyah; Colin Ferguson, the man responsible for the LIRR shootings, who would later reject Kuby & Kunstler's legal counsel and choose to represent himself at trial; Qubilah Shabazz, the daughter of Malcolm X, accused of plotting to murder Louis Farrakhan of the Nation of Islam; Glenn Harris, a New York public school teacher who absconded with a fifteen year-old girl for two months; Nico Minardos, a flamboyant actor indicted by Rudy Giuliani for conspiracy to ship arms to Iran; Darrell Cabey, one of the victims of the Bernard Goetz shooting; and associates of the Gambino crime family. During the first Gulf War, they represented dozens of American soldiers who refused to fight and claimed conscientious objector status. They also represented El-Sayyid Nosair, the assassin of the late Jewish leader Rabbi Meir Kahane.
During the 1994-95 television season, Kunstler starred as himself in an episode of Law & Order titled "White Rabbit". It was based on the 1971 shooting of a policeman in connection with the robbery of a Boston Brinks truck by members of the Weatherman Underground.
In late 1995, Kunstler died in New York of heart failure at the age of 76. In his last major public appearance, at the commencement ceremonies for the University of Buffalo's School of Architecture and Planning, Kunstler lambasted the death penalty, saying, "We have become the charnel house of the Western world with reference to executions; the next closest to us is the Republic of South Africa."
William Kunstler was survived by his wife Margaret Ratner Kunstler and daughters Karin Kunstler Goldman, Jane Drazek, Sarah Kunstler and Emily Kunstler and grandchildren Jessica Goldman, Daniel Goldman and Andrew Drazek. Emily Kunstler and Sarah Kunstler are currently producing a documentary about their father entitled Disturbing the Universe: Radical Lawyer William Kunstler (film) that will be completed in fall of 2008.