While in the Marines, Diaz served as a prosecutor, defense lawyer and judge. He left the service in 1995 for private practice, becoming an associate with the law firm of Hunton & Williams and represented Philip Morris USA during tobacco lawsuits in the late 1990s. From 2000 to 2005, he served as a military judge for the U.S. Navy-Marine Corps Trial Judiciary and as an appellate judge for the U.S. Navy-Marine Corps Court of Criminal Appeals.
Work as a judge in North Carolina
In 2001, then-North Carolina Gov. Mike Easley appointed Diaz to the North Carolina Superior Court, making Diaz the first Hispanic ever to be a state judge in North Carolina. The following year, Diaz lost a bid for election. However, Easley again appointed Diaz to the Superior Court. Then, in 2005, the North Carolina Supreme Court chief justice appointed Diaz to be Charlotte, North Carolina's first ever Business Court judge, one of just three in the state.
Federal judicial service
On November 4, 2009, President Barack Obama nominated Diaz to be a United States circuit judge of the United States Court of Appeals for the Fourth Circuit, to replace Judge William Walter Wilkins, who had taken senior status in July 2007 and later retired. Diaz was nominated to the seat to which Steve A. Matthews previously had been nominated by President George W. Bush. The nomination, made along with that of fellow North Carolina nominee James A. Wynn, Jr., was jointly endorsed by North Carolina senators Kay Hagan, a Democrat, and Richard Burr, a Republican. The Senate Judiciary Committee voted 19–0 on January 28, 2010, to send his nomination to the Senate floor. A combination of secret holds and the threat of filibuster by Republicans caused Democratic Senate Majority LeaderHarry Reid not to bring Diaz's confirmation to a vote for nearly eleven months. On December 18, 2010, the Senate confirmed Diaz by voice vote. He received his commission on December 22, 2010.
Judiciary Committee hearing
Diaz received a unanimous vote from the Senate Judiciary Committee to forward his nomination to the full Senate. He had a hearing before the Committee on December 16, 2009. He was heard along with fellow nominee James Wynn by just three of the Committee members. When asked about his judicial philosophy, Diaz said: "We're not simply dealing with an academic exercise, but we're affecting people's lives in each and every case". The Judiciary Committee reported him to the full Senate on January 28, 2010 and the Senate confirmed his nomination on December 18, 2010.