2014–2015 season

On May 11, 2014, Mori debuted in the Pacific League against the Saitama Seibu Lions as a relief pitcher.^[2] In the match against the Chunichi Dragons in the Interleague play on May 26, he recorded a Hold for the first time.^[3] And in the match against the Hokkaido Nippon-Ham Fighters on July 12, he pitched as a relief pitcher and became his first Winning pitcher.^[4] In 2014 season, Mori recorded with a 58 Games pitched, a 4–1 Win–loss record, a 2.33 ERA, a 20 Holds, a one a 54 strikeouts in 65.2 innings. In the 2014 Japan Series against the Hanshin Tigers, he pitched in Game 1, Game 4, and Game 5 as a relief pitcher, scoring 2 Holds in each inning with no runs.^[5][6][7]

On July 19, 2015, Mori participated the All-Star Game for the first time in MAZADA All-Star Game 2015.^[8] In 2015 season, he recorded with a 55 Games pitched, a 5–2 Win–loss record, a 2.69 ERA, a 16 Holds, a 66 strikeouts in 60.1 innings. In the 2015 Japan Series against the Tokyo Yakult Swallows, he pitched in Game 4 and Game 5 as a relief pitcher, contributing to the team's second consecutive Japan Series champion with no runs in either match.^[9][10]

2016–2020 season

In the match against the Tohoku Rakuten Golden Eagles on May 8, 2016, he recorded a save for the first time.^[11] In 2016 season, he pitched as a setup man, and finished the regular season with a 56 Games pitched, a 4–3 Win–loss record, a 2.98 ERA, a 14 Holds, a one Save, a 51 strikeouts in 60.1 innings.

In 2017 season, Mori finished the regular season with a 64 Games pitched, a 2–3 Win–loss record, a 3.92 ERA, a 33 Holds, a one Save, a 60 strikeouts in 64.1 innings. In the 2017 Japan Series against the Yokohama DeNA BayStars, as a setup man, he pitched in four games, Game 1, Game 2, Game 3, and Game 6, and contributed to the team's first Japan Series champion in two years.^[12]

Mori pitched as a setup man until the 2016 season, but he pitched as a closer pitcher for the reason that Dennis Sarfate out of the team for surgery from the 2018 season. On July 13, 2018, he participated in the mynavi All-Star Game 2018 for the 2rd time.^[13] Mori recorded seven consecutive Saves on the NPB record between September 18 and 25.^[14] He finished the regular season with a 66 Games pitched, a 2–4 Win–loss record, a 2.79 ERA, a 6 Holds, a 37 Saves (League Saves leader), a 61 strikeouts in 61.1 innings. In the 2018 Japan Series against the Hiroshima Toyo Carp, he pitched as a closer and contributed to the team's second consecutive Japan Series champion with a 3 saves in 5 games.^[15] On November 27, Mori was honored for the Pacific League Saves leader Award at the NPB AWARD 2018.^[16]

Mori had recorded 19 saves as a closer since the start of the 2019 season, but left the team on June 16 due to a back muscle injury.^[17] On July 21, he returned to the team and pitched for the rest of the season as a closer.^[18] Mori finished the regular season with a 54 Games pitched, a 2–3 Win–loss record, a 2.21 ERA, a 7 Holds, a 35 Saves, a 59 strikeouts in 53 innings. In the 2019 Japan Series against the Yomiuri Giants, he pitched as a closer and contributed to the team's third consecutive Japan Series champion with a one Save in 4 games.^[19]

In the match against the Chiba Lotte Marines on August 20, 2020, Mori recorded a total of 100 Holds.^[20] And in the match against the Chiba Lotte Marines on October 11, he recorded a total of 100 Saves.^[21] In 2020 season, Mori finished the regular season with a 52 Games pitched, a 1–1 Win–loss record, a 2.28 ERA, a 6 Holds, a 32 Saves, a 40 strikeouts in 51.1 innings. In the 2020 Japan Series against the Yomiuri Giants, he pitched as a closer and contributed to the team's fourth consecutive Japan Series champion with a one Save in 3 games.^[22]

2021–2023 season

On April 29, 2021, Mori was diagnosed with left olecranon bursitis.^[23] The Hawks announced he underwent surgery on May 28 and will require four weeks of hospitalization.^[24] On September 8, he pitched against the Saitama Seibu Lions after four months of medical treatment.^[25] In 2021 season, he finished the regular season with 30 Games pitched, a 1–3 Win–loss record, a 4.03 ERA, a 15 Saves, and a 26 strikeouts in 29 innings.

On September 16, 2022, Mori pitched for the first time as a starting pitcher in his 461st career game. He was recorded as the pitcher who starting pitcher for the first time after playing the most games in NPB.^[26] In 2022 season, he finished the regular season with 29 Games pitched, a 2–4 Win–loss record, a 2.62 ERA, a 3 Holds, a 6 Saves, and a 27 strikeouts in 34.1 innings.

In 2023 season, Mori played as a starting pitcher but only pitched in two games.

On October 22, 2023, the Hawks announced they would release him.^[27]

2024 season–present