Judge Diaz was appointed to the King County Superior Court in December 2017 by Governor Jay Inslee, effective January 22, 2018. Judge Diaz assumed the position previously occupied by the Hon. Richard McDermott, who retired from the bench after nearly 18 years of service. Judge McDermott himself succeeded the Hon. James Bates, who was the first Judge of Department 38.

Judge Diaz immigrated from Peru to Seattle as an infant. His family settled eventually in the then-working-class neighborhood of Ballard and, in middle school, moved down to the White Center/Burien area. His family spoke Spanish exclusively at home. He was the first and remains the only lawyer in his large Latino family.

In 1996, Judge Diaz received his Bachelor of Arts (BA) degree in Philosophy, magna cum laude, from the University of Notre Dame. He then was a graduate student at Princeton University, studying classical philosophy for two years. Judge Diaz received his law degree (JD) from Cornell Law School in 2002, where he was a member of the Cornell International Law Journal.

From 2002 to 2006, Judge Diaz was an associate in the Houston office of the international law firm of Fulbright & Jaworski LLP, litigating complex commercial and white collar criminal matters. Judge Diaz then returned home and joined the litigation boutique Yarmuth Wilsdon Calfo, litigating the same types of cases.

Judge Diaz was an Assistant United States Attorney of the United States Attorney's Office for the Western District of Washington (“DOJ”) from 2008 to 2018, where he founded the office’s Civil Rights Program in 2011. In that capacity, he investigated and/or prosecuted a wide variety of civil rights matters, from "classic" civil rights cases such as housing, employment and educational matters to more "modern" matters, such as disability rights, servicemembers rights, and rights violated by police misconduct. He was the lead line attorney on the United States v. City of Seattle (Seattle PD) consent decree matter from its inception in 2010 until the SPD was found in full and effective compliance in 2018.

Among the honors he received while at DOJ, in 2014, Judge Diaz received the EOUSA Director's Award for “extraordinary professional achievements and excellence.” He also received the Thomas C. Wales Performance Award, the highest award given at the U.S. Attorney's Office, and the Assistant Attorney General for Civil Rights Division's Distinguished Service Award, both in 2012. Judge Diaz was on faculty regularly between 2012 and 2017 at the DOJ's National Advocacy (Training) Center, lecturing on civil rights enforcement.

In 2016, President Obama nominated Judge Diaz to be a U.S. District Court Judge for the Western District of Washington. The nomination was returned to the President in January 2017, at the end of the 114th Congress, without Senate action.

As an attorney, Judge Diaz regularly volunteered at the King County Bar Association's Neighborhood Legal Clinic Program's Spanish Language Legal Clinic, which he later helped merge with the El Centro de La Raza clinic. He served on KCBA's Pro Bono Service Committee, was Chair of its Neighborhood Legal Clinic Program's Advisory Committee, and Chair of its Spanish Clinic Subcommittee.

Judge Diaz is a Commissioner of the Washington State Supreme Court’s Minority and Justice Commission, and volunteers at his daughters' elementary school, including as a soccer coach/team manager and moot court judge.