Mendoza was born in 1971 in Pacoima, California,[3] to parents who immigrated to the United States from Mexico.

[3] While in law school, he spent one summer as a legal intern for the United Farm Workers of America.

[8] Mendoza ran for a vacant seat on the superior court for Benton and Franklin counties in 2008, but lost the election.

[7][3] On January 16, 2014, President Barack Obama nominated Mendoza to serve as a United States district judge for the Eastern District of Washington, to the seat vacated by Judge Lonny R. Suko, who assumed senior status on November 1, 2013.

[9] Mendoza's name was forwarded to Obama by Senator Patty Murray upon the recommendation of a bipartisan committee of eight that reviewed candidates for the Eastern District of Washington.

President Biden nominated Mendoza to the seat to be vacated by Judge M. Margaret McKeown, who announced her intent to assume senior status upon confirmation of a successor.

[20] On November 13, 2023, Mendoza was in a 7–4 majority that temporarily blocked Idaho's abortion ban due to its lack of exceptions for medical emergencies.

The Supreme Court later vacated its stay in Moyle v. United States, returning the case back to the 9th Circuit en banc panel.