[2] Diaz also served with the Marines from 1988 to 1995 as a judge advocate, retiring as a lieutenant colonel, USMCR.

[1] 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.

[6] 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 assumed senior status in July 2007 and later retired.

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".

[11] A combination of secret holds and the threat of filibuster by Republicans caused Democratic Senate Majority Leader Harry Reid not to bring Diaz's confirmation to a vote for nearly eleven months.

[5] In August 2020, Diaz wrote for the unanimous panel when it upheld the convictions of Rise Above Movement rioters at the 2017 Unite the Right rally in Charlottesville, Virginia because any unconstitutionally overbroad elements of the Anti-Riot Act were fully severable.