Born in Chicago, Cohen graduated from Amherst College in 1963. He then earned a law degree from the University of California, Berkeley, before beginning his formal legal career. During his university years, he was active in the free speech movement and anti-Vietnam War protests.
In 1967, his work at the California Rural Legal Assistance office attracted the attention of César Chávez. Cohen became Chávez's personal attorney and general counsel of the UFW, directing the UFW legal department for the next 14 years. Cohen and his team of lawyers won numerous suits directed by and against the union including fights against using trespass laws to thwart union organizing. He negotiated an influential jurisdictional accord with the Teamsters that ended years of struggle between the two unions. He negotiated important contracts with growers. He also worked closely with Gov. Jerry Brown in creating the landmark California Agricultural Labor Relations Act of 1975, which extended to farm workers the collective bargaining rights from which they were excluded under the federal National Labor Relations Act of 1935.
He resigned from the UFW in November 1980. Cohen continued to advise and serve as legal counsel to a number of organizations and causes, especially Neighbor to Neighbor, a group founded by the son of Fred Ross that organized key constituencies to pressure members of Congress on progressive legislation. The group used the house meeting tactic developed by Ross that proved so successful during the years in which Cohen was general counsel of the UFW.
In 1995, The Los Angeles Times acquired declassified documents detailing the FBI's surveillance of Chávez between 1965 and 1973. In a quote from the newspaper, Cohen characterized the agency's efforts to uncover communism within the ranks of the UFW as a "witch hunt and an exercise in guilt by association."
Pawel, Miriam. The Union of Their Dreams: Power, Hope, and Struggle in Cesar Chavez’s Farm Worker Movement. New York: Bloomsbury, 2009.