[3] Villarreal began refereeing at the age of 18, when a league in which he played in Brownsville, Texas, needed volunteers for under-5 matches.
He then took a United States Soccer Federation (USSF) course to qualify for grade 8 as a referee and began officiating around Texas.
[4] After working as a fourth official in Major League Soccer (MLS), Villarreal was referee for his first game in the competition on May 24, 2012, between the New York Red Bulls and Chivas USA.
[5] He officiated in the 2014 U.S. Open Cup Final, which Seattle Sounders FC won against the Philadelphia Union.
[6] In the 2015 MLS Cup Playoffs, he admitted after the game to making an incorrect decision in allowing Dilly Duka of the Montreal Impact to score from an offside position against the Columbus Crew, who nonetheless won in overtime.