De León played college baseball at Southern University from 2011 to 2013. He was drafted by the Los Angeles Dodgers in the 24th round of the 2013 Major League Baseball Draft. He signed with the Dodgers and made his professional debut with the Arizona League Dodgers. He was later promoted to the Ogden Raptors. De León returned to Ogden to start the 2014 season. After recording a 2.65 earned run average and 77 strikeouts in innings, he was promoted to the Great Lakes Loons. In his second start with the Loons, De León had 14 strikeouts, breaking Clayton Kershaw's team record of 12 set in 2007. In four starts with Great Lakes, he had a 1.19 ERA with 42 strikeouts in innings. After the season, he was named the Pioneer League Pitcher of the Year, for his performance that season with Ogden. De León was assigned to the Rancho Cucamonga Quakes of the California League to start the 2015 season. In seven starts for the Quakes he was 4–1 with a 1.67 ERA and he was promoted to the AA Tulsa Drillers of the Texas League on May 18. He was selected to the California League mid-season all-star team but was unable to participate because of his promotion to AA. He was selected as a second teamBaseball Americaminor league all-star after the season. He had a 2–6 record in AA with a 3.64 ERA and 105 strikeouts. He was given a non-roster invitation to Dodgers spring training. In 2016, he spent the season in AAA with the Oklahoma City Dodgers, where he was 7–1 with a 2.61 ERA in 16 starts. He also struck out 111 batters against only 20 walks. De León had his contract purchased by the Dodgers and was called up to the majors on September 3, 2016, making his debut as the starting pitcher for the Dodgers against the San Diego Padres on September 4. He allowed four runs in six innings in his debut to pick up the win. His nine strikeouts was the second most by an Los Angeles Dodgers pitcher in his debut, trailing only Pedro Astacio and Kazuhisa Ishii. In four starts for the Dodgers he was 2–0 with a 6.35 ERA.
Tampa Bay Rays
On January 23, 2017, the Dodgers traded De León to the Tampa Bay Rays in exchange for second basemanLogan Forsythe. De León started the 2017 season ranked as the Rays' third best prospect and 28th overall by MLB Pipeline. He opened the season on the minor league disabled list due to discomfort in the flexor mass muscle in the forearm, which is a precursor to Tommy John Surgery. On May 29, De León got his first call-up by the Rays due to a depleted bullpen from a 15 inning victory against the Minnesota Twins the day before. In his Rays debut, he allowed three runs on four hits, two walks, and two strikeouts in 2 innings pitched, and took the win. He was sent back down the next day. On June 9, De León was put on the minor league disabled list with a mild lat strain. On August 14, De León was placed on the minor league disabled list for the third time in 2017, this time with elbow tendinitis. De León was diagnosed with a torn ulnar collateral ligament on March 7, 2018. One week later, Dr. James Andrews successfully performed Tommy John surgery, ending De León's 2018 season. De León returned to game action in May 2019. He returned to the Rays active roster on August 14.
Cincinnati Reds
On November 20, 2019, De León was traded to the Cincinnati Reds in exchange for cash considerations or a PTBNL.
