Dominic Leone

High school and college

Leone attended Norwich Free Academy in Norwich, Connecticut, where he played for the school's baseball team. Undrafted out of high school, he then enrolled at Clemson University, where he pitched for the Clemson Tigers baseball team.

As a true freshman, the Tigers turned to Leone in a game where they faced postseason elimination; Leone won the game, which sent the team to the 2010 College World Series. A strained shoulder in his sophomore year led him to develop his off-speed pitches. In his junior year, he had a 7–4 win–loss record and a 5.25 earned run average (ERA). While strugging in his junior year, Leone taught himself to throw a cutter by watching YouTube videos of Mariano Rivera.

Seattle Mariners

Leone was selected by the Seattle Mariners in the 16th round of the 2012 Major League Baseball draft, and received a $100,000 signing bonus. The Mariners assigned him to the Short Season-A Everett AquaSox, where he made 19 appearances and posted a 3–0 record, 1.36 ERA, 39 strikeouts in 33 total innings. In 2013, Leone began the season with the Class-A Clinton LumberKings. After pitching  6 ¹⁄₃ scoreless innings for Clinton, the Mariners promoted Leone to the Advanced-A High Desert Mavericks of the California League, and served as their closer. In July, Leone was promoted to the Double-A Jackson Generals, where he ended the 2013 season. In 48 combined appearances across three minor league levels, Leone posted a 1–3 record, 2.25 ERA, and 64 strikeouts in 64 innings. 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.

On April 4, 2014, the Mariners selected Leone's contract from the Triple-A Tacoma Rainiers of the Pacific Coast League, promoting him to the major leagues. 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 66 ¹⁄₃ innings.

During spring training in 2015, Leone struggled with his command, and the Mariners assigned him to Triple-A Tacoma. When the Mariners placed Tom Wilhelmsen on the disabled list in early April, they promoted Leone. Leone made 10 appearances with the Mariners in 2015, posting a 6.35 ERA and nine walks in 11 ¹⁄₃ innings. In the minors, he put up a 1–1 record, 7.71 ERA, and eight strikeouts in 9 ¹⁄₃.

Arizona Diamondbacks

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. Leone made three appearances for the Diamondbacks in 2015, and went 0–1 with a 14.73 ERA in 3 ²⁄₃ innings. With the Double-A Mobile BayBears, Leone made 19 appearances and pitched to a 1–2 record, 3.90 ERA, and 28 strikeouts in 27 ²⁄₃ innings. In 2016, Leone split time between the Diamondbacks and the Triple-A Reno Aces. With Arizona he posted a 0–1 record, 6.33 ERA, and 23 strikeouts in 27 total innings. With Reno, Leone went 5–2 in 33 relief appearances, and recorded a 3.34 ERA and 36 strikeouts in 35 innings. In November 2016, Leone was designated for assignment by the Diamondbacks.

Toronto Blue Jays

On November 18, 2016, Leone was claimed off waivers by the Toronto Blue Jays. Though initially sent by the Jays to the minor league camp late in spring training, Leone was recalled on April 2 to start the 2017 season with the major league club, taking the roster spot of the injured closer Roberto Osuna. He was optioned to the Triple-A Buffalo Bisons on May 30 but was recalled again on June 6. Leone was placed on the bereavement list for undisclosed reasons on August 13, and activated on August 16. 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.