In 1967 Nix was elected as a judge on the Philadelphia Court of Common Pleas. He was appointed an associate justice of the Pennsylvania Supreme Court by Governor Milton Shapp in 1971, and was elected the following year. He was the first African American elected to statewide office in Pennsylvania history. Of his election, Justice Nix said: ""t shows that the people want ideas, that they are not interested in race, creed or color. I'm particularly impressed by the vote in the central counties. It is just unbelievable that a Philadelphia candidate won there. And a black man!"" He became Chief Justice in 1984, replacing former Chief Justice Samuel J. Roberts. From 1991 to 1992 he served as President of the National Conference of Chief Justices.