Cincinnati Reds The Reds selected Stephenson in the first round of the
2011 Major League Baseball (MLB) draft, with the 27th overall selection. Stephenson became the Reds' first high school player chosen in the first round of the
MLB draft since they chose catcher
Devin Mesoraco in the first round of the
2007 draft, and the first
prep school pitcher the organization had chosen in the first round since the selection of
Homer Bailey in the
2004 draft.
Baseball America rated Stephenson as one of the Reds' top ten prospects before the 2012 season, despite his not having made his professional debut. Stephenson began the 2012 season in
extended spring training. Stephenson made his professional debut with the
Billings Mustangs of the Rookie-level
Pioneer League on June 19, 2012. He pitched five innings and struck out eight batters. In seven games with Billings, Stephenson had a 2.05 ERA, allowing only 22 hits and eight walks in innings pitched, while striking out 37. In July, the Reds promoted Stephenson to the
Dayton Dragons of the
Single–A Midwest League. For Dayton, Stephenson had a 4.19 ERA in eight starts, as he allowed 32 hits and 15 walks, while striking out 35 batters. After pitching to a 5–3 win–loss record and a 2.57 ERA with 96 strikeouts in 77 innings, he was promoted to the
Bakersfield Blaze of the
High–A California League on July 18. Less than a month later, after pitching to a 2–2 record and a 3.05 ERA with Bakersfield, the Reds promoted Stephenson to the
Pensacola Blue Wahoos of the
Double–A Southern League in August, The Reds invited Stephenson to
spring training as a non-roster invitee in 2014. The Reds assigned Stephenson to Pensacola. He appeared in the 2014
All-Star Futures Game. He had a 7–10 record and a 4.74 ERA while striking out 140 batters in innings pitched with Pensacola. The Reds invited Stephenson to spring training in 2015. He began the season with Pensacola, where he pitched to a 3.68 ERA with 89 strikeouts in innings, including a 2.36 ERA in his last ten starts, before he was promoted to the
Louisville Bats of the
Triple–A International League. After the season, the Reds added him to their 40-man roster. After competing for a spot in the Reds'
starting rotation for
Opening Day during spring training in 2016, the Reds
optioned him to Louisville to start the season. On April 4, with
Homer Bailey beginning the season on the
disabled list the Reds added Stephenson to their Opening Day roster. He made his MLB debut on April 7, and was optioned back to Louisville after the start. Stephenson was called up April 19, to make his second start in place of right-hander
Alfredo Simon. In his second start, against the
Colorado Rockies Stephenson became the first Reds pitcher to complete seven innings up to that point in the 2016 season, allowing just three hits and one earned run while striking out three and walking two. After the game, Stephenson was optioned back to Louisville. In 8 starts for the Reds, he finished 2–3 with a 6.08 ERA in 37 innings. On July 22, 2017, Stephenson was recalled from Louisville. For the season, in 25 games, 11 starts, Stephenson finished 5–6 with a 4.68 ERA in innings. He struck out 86 but walked 53 batters. He spent the majority of the 2018 season with Louisville, only making four appearances for Cincinnati towards the end of the season. In 2019, Stephenson was used as a
relief pitcher He appeared in 57 games, all out of the bullpen, and enjoyed career bests in ERA (3.76), WHIP (1.036), K/9 (11.3), BB/9 (3.3), and H/9 (6.0). Stephenson attributed his recent success to letting go of expectations and focusing on the present. In 2020, Stephenson recorded a 9.90 ERA with 13 strikeouts in 10 innings pitched, as eight of his 11 hits allowed were home runs.
Colorado Rockies On November 25, 2020, Stephenson and Jameson Hannah were traded to the
Colorado Rockies in exchange for
Jeff Hoffman and Case Williams. In his first season in Colorado, Stephenson posted a 3.13 ERA in 49 games. He struck out 52 batters in 46 innings. He had a 6.04 ERA for the Rockies in 2022, with a 10.38 ERA in eight appearances in August, and was designated for assignment on August 25.
Pittsburgh Pirates The
Pittsburgh Pirates claimed Stephenson off of
waivers on August 27, 2022. He had a 3.38 ERA in innings with the Pirates.
Tampa Bay Rays On June 2, 2023, the Pirates traded Stephenson to the
Tampa Bay Rays in exchange for
Alika Williams. In 42 appearances out of the bullpen, he registered a 2.35 ERA with 60 strikeouts across innings of work. Stephenson became a free agent following the season.
Los Angeles Angels On January 23, 2024, Stephenson signed a three-year, $33 million contract with the
Los Angeles Angels. On April 18, it was announced that Stephenson would miss the entirety of the season due to an undisclosed elbow injury. The following week, the injury was revealed to be a
UCL injury that necessitated
Tommy John surgery. Stephenson returned to action in 2025, making rehab appearances for the Triple-A
Salt Lake Bees and Single-A
Inland Empire 66ers. On May 28, 2025, Stephenson was activated from the injured list to make his Angels debut. He made two scoreless appearances for Los Angeles before being placed back on the injured list with right biceps inflammation on June 2. Stephenson was transferred to the 60-day injured list on July 24. He was activated on August 22. In 12 appearances for Los Angeles, Stephenson logged a 2-0 record and 2.70 ERA with 10 strikeouts over 10 innings of work. On September 20, Stephenson was placed on the injured list due to right elbow inflammation, officially ending his season. On March 14, 2026, it was announced that Stephenson had suffered a setback during spring training pitch sessions, which included possible damage to his UCL. On April 8, it was announced that Stephenson had undergone a season-ending ligament and flexor repair surgery. ==Personal life==