Diaz practiced law on the Gulf Coast. He served in the Mississippi House of Representatives from District 116 representing Biloxi and D'Iberville for seven years from 1988 to 1994. During his tenure in the Legislature, he was a subcommittee Chairman for the Insurance Committee and for the Judiciary Committee. He was on the Ways and Means Committee and was Secretary for the Constitution Committee. He also served as City Attorney for the City of D'Iberville for four years. Justice Diaz was elected to the Court of Appeals in November 1994 and served in that position until March 2000, at which time he was appointed to the Supreme Court by Governor Ronnie Musgrove. In 2000, he was elected to the Supreme Court for an eight year term beginning January 2001.