Domínguez was born in Esperanza Valverde Mao, Dominican Republic. As to where his unusual first name comes from, Baseball America reported: "'I’m not sure where it comes from,' Seranthony Domínguez said with a laugh. 'I think it’s something my parents heard on TV.'"
Career
Minor leagues
Domínguez signed with the Philadelphia Phillies as an international free agent in October 2011 for $25,000. He made his professional debut in 2012 with the DSL Phillies and spent the whole season with them, going 4-4 with a 3.48 ERA and 40 strikeouts in 67.1 innings in 15 games. In 2013, he returned to the DSL where he compiled a 4-6 record and 2.96 ERA, and 58 strikeouts in 76 innings, in 14 starts. He led the Dominican Summer League in shutouts and complete games. In 2014, he played for the GCL Phillies, for whom he pitched to a 2-2 record and 3.12 ERA with 24 strikeouts in 26 relief innings pitched. He spent 2015 with the GCL Phillies, where he was 1-1 with a 2.35 ERA with 9 strikeouts in only 7.2 innings pitched. In 2016 he pitched for both the Class A Short-SeasonWilliamsport Crosscutters and Class ALakewood BlueClaws, with whom Domínguez had a combined 6-3 record and 2.34 ERA in 13 starts with 65 strikeouts in 65.1 innings. In 2017 he pitched for the Class A-Advanced Clearwater Threshers, and compiled a 4-4 record and 3.61 ERA with 75 strikeouts in 62.1 innings in 15 games, averaging 10.8 strikeouts per 9 innings. He was a 2017 Florida State League Mid-Season All Star with Clearwater. The Phillies added him to their 40-man roster after the 2017 season. In the 2017 off-season, the Phillies organization told him he was being converted into a relief pitcher. He began 2018 with the Double-A Reading Fightin Phils, going 1-2 with a 2.08 ERA with 18 strikeouts in 13 innings in 8 games, averaging 12.5 strikeouts per 9 innings, and was promoted to the Triple-A Lehigh Valley IronPigs in late April for whom he was 1-0 with a 0.00 ERA in 3 games. Between the two minor league teams, in 16.2 innings Domínguez gave up 8 hits and 2 walks, as he had 21 strikeouts.
Major leagues
Domínguez was promoted to the Phillies on May 7, 2018, and made his MLB debut the same night. He became the only major league reliever in recorded baseball history to record at least two outs and allow no runs, hits, or walks in his first five major-league appearances. He also became the first pitcher in recorded history to not allow a run, hit, or walk in his first six appearances. He recorded his first career save against the St. Louis Cardinals on May 19, and his first win on May 26. Domínguez tied the all-time record for most perfect appearances in a pitcher’s first 11 games, and pitched more innings in those games than any of the other pitchers with whom he was tied. His 0.65 WHIP through the All-Star break was the lowest ever by any Phillie. In 2018 with the Phillies, he was 2-5 with 16 saves and a 2.95 ERA, and 74 strikeouts in 58 innings, averaging 11.5 strikeouts per 9 innings. Dominguez ranked in the top 25 in major league baseball in both strikeout rate and in ground-ball rate. He struck out 32% of batters, best among rookies with at least 55 innings pitched. He also limited all batters to a.156 batting average and 0.93 WHIP, and batters to a.163 batting average in “high-leverage” situations, as calculated by Baseball-Reference. Batters had a 66.4% contact percentage against him, second-lowest in the NL among pitchers with at least 50 innings pitched. His 16 saves tied Jack Meyer for the most by any Phillies rookie reliever. Baseball America named him to its 2018 MLB All-Rookie Team, and he was named to Baseball Digest's Rookie All-Star team. In 2019 he was 3-0 with a 4.74 ERA in 24.2 innings over 27 relief appearances during which he struck out 29 batters, averaging 10.6 strikeouts per 9 innings. His season ended early with him on the injured list, due to a damaged ulnar collateral ligament in his right elbow. Dominguez underwent Tommy John surgery on July 30, 2020.
Pitching style
Domínguez throws a fastball that can reach 98-99 mph at times, an 89 mph slider that in 2018 generated a whiff rate above 50%, and a change-up. In 2018, batters hit.067 against his slider, with 34 strikeouts in 64 plate appearances.
