Draft and minor leagues The
Oakland Athletics selected Treinen in the seventh round of the
2011 MLB draft. He signed, receiving a $52,000
signing bonus. In January 2013, the Athletics traded Treinen to the
Washington Nationals in a three-team deal, in which Washington also received
A. J. Cole and
Ian Krol, Oakland received
John Jaso, and the
Seattle Mariners received
Michael Morse. With the
Harrisburg Senators of the
Double-A Eastern League, Treinen had a 3.64 ERA in 2013. He began the season with the
Syracuse Chiefs of the
Triple-A International League and was promoted to the major leagues on April 12. He earned his first career win on June 29, making a spot start against the
Chicago Cubs and out-dueling
Jeff Samardzija in a 7–2 win. During the
2014 season, Treinen appeared both as a starting pitcher and reliever, with his fastball reaching . Splitting the season between the Nationals and Triple-A, Treinen had a 2.49 ERA in 15 major league games and a 3.35 ERA in 16 minor league games. Treinen started the
2015 season in the Nationals bullpen, with the offseason signing of
Max Scherzer creating a crowded starting rotation. Manager
Matt Williams, dealing with injuries to his veteran relievers, began using Treinen in late-game situations in early April, occasionally
setting up for closer
Drew Storen. He also reached with his
sinker for the first time in his major league career in April, throwing his fastest career MLB pitch in May at . Treinen remained with the Nationals for much of the year, posting a 2–5 record with 3 blown saves, a 3.85 ERA and 65 strikeouts for the season. He was optioned back to Triple-A on June 20 after struggling with command for the first half of the season. Upon being recalled the next month, Treinen told
The Washington Post that being sent down "was probably the best thing that could have happened", allowing him to work on his approach. However, even late into the season, Treinen struggled to retire left-handed batters, ultimately giving up the final run of the Nationals' season on a solo home run by left-handed
New York Mets outfielder
Curtis Granderson in a 1–0 loss to the eventual
NL champions on October 4. In
2016, Treinen posted a 2.28 ERA on the year and narrowed his platoon splits versus left-handed batters. He credited veteran teammate
Matt Belisle and the team's new pitching coach,
Mike Maddux, for helping him improve, while Maddux described Treinen's progress over the course of the season as "puppy dog to bulldog". Treinen led the
National League in groundballs induced with 65.9% on the season and ranked highly in inherited runners stranded and soft contact created, among other metrics. Treinen was frequently used in situations when another Nationals pitcher had allowed one or more baserunners and manager
Dusty Baker was seeking a
double play ball, a role in which he excelled with his high-90s
sinker. He was exclusively a reliever for the Nationals all season and made his first playoff appearance for the team, being credited with the win in Game 2 of the
National League Division Series (NLDS) against the
Los Angeles Dodgers and taking the loss in Game 4. He had a 6.75 ERA in his first postseason. With the departure of free agent
Mark Melancon, the Nationals named Treinen their
closer to start the
2017 season. On
Opening Day, Treinen pitched a perfect ninth inning against the
Miami Marlins with two strikeouts, earning the save, the second of his career. That performance was the only clean inning Treinen pitched in April. He gave up an earned run while notching his second save of the season on April 5 against the Marlins before blowing a save against Miami the following day. On April 19, Baker removed Treinen, sporting a 7.11 ERA, from the closer role, demoting him in favor of
Shawn Kelley and later
Koda Glover. Despite his poor results, Treinen was called one of the toughest pitchers in the
NL East by reigning MVP
Christian Yelich. In June, Treinen's
batting average on balls in play hit off him gradually lowered. However, he squandered his first chance in months to earn a save: with both Kelley and Glover on the disabled list and relievers
Enny Romero and
Matt Albers unavailable, acting manager
Chris Speier, needing someone to close out the ninth inning of a 4–2 game against the Chicago Cubs on June 29, called on Treinen for his first save opportunity since being demoted from the closer role. Treinen gave up three earned runs for a blown save as the Cubs won 5–4. The Nationals used Treinen only three more times before trading him.
Oakland Athletics (2017–2019) On July 16, 2017, the Nationals traded Treinen back to the
Athletics, along with
Sheldon Neuse and
Jesús Luzardo, seeking to upgrade their bullpen with relievers
Sean Doolittle and
Ryan Madson. At the time of the trade, Treinen was 0–2 with a 5.73 ERA, 3 saves, and two blown saves. With the Athletics, Treinen blew two more saves before picking up a save. In the first half of the
2018 season, Treinen had a 5–1 record with an 0.79 ERA and 23 saves. He was named an
All-Star. He finished the season with a 9–2 record, 38 saves (third in the AL), 3 blown saves and an 0.78 ERA. He gave up three runs in the
Wild Card Game, including a home run to
Giancarlo Stanton. in 2019|left|498x498pxTreinen won his
salary arbitration case before the 2019 season, getting a $6.4 million salary for the season. On June 21, 2019, Treinen was placed on the 10-day
injured list with a right shoulder injury after giving up three runs to the
Tampa Bay Rays without recording an out. He lost the closer role to
Liam Hendriks. Treinen was activated on July 3 and earned a loss that night. In 2019, Treinen was 6–5 with a 4.91 ERA, 16 saves, 5 blown saves, and 59 strikeouts in innings. On December 2, Oakland non-tendered Treinen, opting not to pay him a projected $8 million for the next season, and he became a
free agent.
Los Angeles Dodgers (2020–present) On December 15, 2019, Treinen signed a one-year, $10 million contract with the
Los Angeles Dodgers. The
2020 season was shortened by the
COVID-19 pandemic, but Treinen appeared in 27 games for the Dodgers (third most in the NL), with a 3–3 record with one save in two chances and a 3.86 ERA. In the postseason, he pitched one inning in the
Wild Card series, innings in the
NLDS, innings in the
National League Championship Series (NLCS) (earning a loss in Game 1), and innings in the
World Series, allowing six runs during the postseason and picking up a save in Game 5 of the World Series. In January 2021, the Dodgers re-signed Treinen to a two-year, $17.5 million contract, which also included a team option for a third year. At the start of the
2021 season, he threw his first 100 mph pitches since 2018. In the playoffs, he pitched scoreless innings in the
Wild Card Game, allowed one run on one hit with five strikeouts in innings in the
NLDS, and gave up one run in innings in the
NLCS. Treinen had a sore shoulder early in the 2022 season that shut him down for the first half of the season. Despite the injury, on May 22, the Dodgers signed him to a one-year, $8 million, extension covering the 2023 season and included a conditional option for 2024. He pitched in five games for the Dodgers, allowing one run in five innings On November 11, Treinen underwent right shoulder labrum and rotator cuff repair surgery, with an estimated 10-month recovery. He did not pitch in 2023 but the team picked up his $1 million option for 2024. Treinen was expected to return for the start of the
2024 season but suffered cracked ribs and a bruised lung in
spring training and began the season on the injured list. After a minor league rehab assignment, he was activated by the Dodgers on May 5. He was effective despite lower velocity. He pitched scoreless innings with two saves in the
NLDS and allowed one run in innings in the
NLCS. He earned two wins with a 4.15 ERA in innings in the
World Series, pitching poorly in the 9th inning of Game 2. Treinen earned his first loss and blown save of the 2025 season on April 6. On April 19, he returned to the injured list due to forearm tightness; he was transferred to the 60-day injured list on May 1. Treinen rejoined the Dodgers on July 27, and he allowed one run in of an inning the following day. In September, Treinen earned the loss in five consecutive Dodgers losses, a feat not accomplished in the majors since at least 1912. The streak started on September 6, when he relieved
Yoshinobu Yamamoto, who had pitched no-hit innings. The fifth loss was on September 21, when he allowed three runs and got two outs after
Emmett Sheehan's scoreless seven innings. He finished the season with a 2–7 record, 5.40 ERA in innings over 32 games. His struggles continued in the postseason as Treinen allowed two runs in one inning of the
2025 NLDS, one run in innings of the
2025 NLCS and one run on five hits in innings of the
2025 World Series. ==Personal life==