Born in Ciudad Juárez, Mexico, Mendiola joined the LA Galaxy Academy in 2009. While at the academy level Mendiola won back-to-back U-16 Development Academy Player of the Year awards after the 2009–10 and 2010–11 seasons.
On February 20, 2014 it was announced that Mendiola was signed as a homegrown player by the Galaxy. He then made his professional debut with the LA Galaxy II, the reserve side of the LA Galaxy, on March 22, 2014 in the sides first ever USL Pro match against the Orange County Blues. His first appearance with the LA Galaxy first team came on May 17, 2014 in a regular season MLS match against the Houston Dynamo.