As an assistant to Thurgood Marshall, he represented the National Association for the Advancement of Colored People before courts in criminal cases throughout the American South.
In 1950, he was appointed director of the NAACP's western region, where he directed drives involving open housing, school desegregation and civil rights.
Under President Johnson, Williams became the first black representative to the United Nations Economic and Social Council and later was appointed ambassador to Ghana.
During his three-year tenure at this post, he was credited with improving the formerly strained relations between the United States and the African nation.
For 20 years, Williams was president of the Phelps Stokes Fund, established to facilitate the education of African and Native American students.