León made his professional baseball debut as a 17-year-old with the Saraperos de Saltillo of the Mexican League in 2006, pitching in 4 games as a reliever. He appeared in 35 games for Saltillo in 2007, posting a 3–0 win–loss record, 1.94 earned run average, and 38 strikeouts in 41 innings pitched. He then played in 30 games for the Tomateros de Culiacán of the Mexican Pacific League, pitching to a 6–0 record, 3.49 ERA, and 27 strikeouts in 28 innings.
Oakland Athletics
The Oakland Athletics purchased León's contract before the start of the 2008 minor league season, and he was assigned to the Advanced-A Stockton Ports. He made 20 relief appearances for Stockton in 2008, posting a 2.86 ERA and 28 strikeouts in 28 innings. León was assigned back to Saltillo on June 24, 2008, and played the rest of the minor league season there, going 2–1 with a 4.30 ERA in 13 games. He again played winter ball with the Tomateros de Culiacán, pitching 20 innings over 23 relief appearances. In 2009, León was assigned to the Double-AMidland RockHounds for the entire minor league season. In 33 appearances, 7 of which were starts, he would post a 2–3 win–loss, 3.51 ERA, and 63 strikeouts in a then career-high 74 innings pitched. For the third consecutive season, León returned to the Mexican winter league, and pitched to a 2–0 record with a 4.15 ERA in 23 appearances. In 2010, he would pitch in 3 games for Midland before being placed on the disabled list, and would then undergo Tommy John surgery that would sideline him for the rest of the season. Having recovered from surgery, León made 5 starts for the Arizona League Athletics in 2011, posting an 8.53 ERA in 6 innings. He began the 2012 season with Stockton, later being promoted to Double-A Midland and the Triple-ASacramento River Cats. In 44 total appearances, León would post a 4–1 win–loss record, 2.70 ERA, and 74 strikeouts in 66 innings. León played for the Mexican national baseball team in the 2013 World Baseball Classic. In a first-round game against the Canadian national baseball team, he initiated a brawl after hitting Rene Tosoni with a pitch following a buntsingle by Chris Robinson. León was converted into a full-time starter during the 2013 minor league season, which he split between Double-A and Triple-A. In 25 total starts, he pitched to a 9–8 record, 4.12 ERA, and 97 strikeouts in 144 total innings. León played the entire 2014 season with Triple-A Sacramento, posting a 10–7 record, 4.97 ERA, and 128 strikeouts in 145 innings pitched. He was briefly promoted to the Major Leagues in May 2014, but returned to Triple-A without having made his MLB debut. León made his Major League Baseball debut on April 22, 2015, through he would spend the majority of the season with the Nashville Sounds, which became Oakland's Triple-A affiliate following the 2014 season. In 20 appearances for Nashville, León pitched to a 2–5 record, 2.95 ERA, and 55 strikeouts. In 19 relief appearances for Oakland, León would post a 0–2 record, 4.39 ERA, and 19 strikeouts.
