The Seattle Mariners selected Leone in the 16th round of the 2012 Major League Baseball draft, and he received a $100,000 signing bonus when he signed with the team.
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.
The Seattle Mariners selected Leone in the 16th round of the 2012 Major League Baseball draft, and he received a $100,000 signing bonus when he signed with the team. 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.
Dominic Leone