[6] While struggling in his junior year, Leone taught himself to throw a cutter by watching YouTube videos of Mariano Rivera.

After he pitched 6+1⁄3 scoreless innings for Clinton, the Mariners promoted Leone to the High–A High Desert Mavericks of the California League, where he served as their closer.

In 48 combined appearances across three minor league levels, Leone posted a 1–3 record, 2.25 ERA, and 64 strikeouts in 64 innings.

[8] In the offseason, he made 11 relief appearances for the Peoria Javelinas of the Arizona Fall League, and recorded a 3.00 ERA and 15 strikeouts.

[10] He remained with the Mariners for the entire 2014 season, making 57 relief appearances and posting an 8–2 record, 2.17 ERA, and 70 strikeouts in 661⁄3 innings.

[8] On June 3, 2015, Leone, along with Welington Castillo, Gabby Guerrero, and Jack Reinheimer, was traded to the Arizona Diamondbacks for Mark Trumbo and Vidal Nuño.

[11] With the Double-A Mobile BayBears, Leone made 19 appearances and pitched to a 1–2 record, 3.90 ERA, and 28 strikeouts in 272⁄3 innings.

[21][22] On September 11, Leone earned his first major league save when he pitched a scoreless 9th inning in a 4–3 victory against the Baltimore Orioles.

[23] Leone would finish his first season as a Blue Jay appearing in 65 games, posting a 2.56 ERA, and striking out 81 batters over 701⁄3 innings.

[11] Leone qualified for super two status during the 2017-18 offseason, and signed a one-year, $1.085 million contract for the 2018 season on January 12, 2018.

[24] On January 19, 2018, the Blue Jays traded Leone and Conner Greene to the St. Louis Cardinals for outfielder Randal Grichuk.

[35] On October 3, 2021, Leone struck out Eric Hosmer of the San Diego Padres to end the last game of the season and clinch the NL West division championship for the Giants.

[39] He made 8 appearances for the Triple-A Round Rock Express, working to a pristine 1.59 ERA and 2–0 record with 15 strikeouts and 2 saves in 11.1 innings pitched.

[41] After hours signing with the Mets, Leone's first game back was against the Detroit Tigers where he pitched in relief for one inning.

[45] On March 27, the White Sox selected Leone's contract after he made the team's Opening Day roster.

[49] After five more games for Chicago, Leone was placed back on the 60–day injured list with a UCL sprain on August 16, ending his season.