An NAACP member who became a Supreme Court justice was Thurgood Marshall.
He was a judge of the Supreme Court of the United States from October 1967 until October 1991, the first African-American elected to the position. Before becoming a judge he had a successful career as a lawyer, with a high success rate before the Supreme Court (he was the one who took the most cases before that instance) and known for his victory in the Brown case against the Board of Education. He was then appointed by President John F. Kennedy to work on the Federal Court of Appeals for the Second Circuit, and in 1965 President Lyndon Johnson appointed him first attorney general and then promoted him to the Supreme Court of the United States.