The Reds drafted Stephenson out of high school, and he chose to forego his college baseball commitment to begin his professional career in their farm system.

He made his MLB debut that year, hitting a home run in his first major league plate appearance, but was used rarely, as the Reds had two catchers.

When Curt Casali left the team prior to the 2021 season, Stephenson became Tucker Barnhart's backup catcher, and he also received significant time as a pinch hitter.

[17] He batted .347 in 13 games for the Glendale Desert Dogs, with seven doubles and three RBI, and received the Dernell Stenson Sportsmanship Award as the player who "best exemplifies unselfishness, hard work, and leadership" in the Arizona Fall League.

[3][20] The Reds sustained a number of injuries and illnesses at the beginning of their pandemic-abbreviated season, and Stephenson was promoted to make his MLB debut on July 26, 2020.

Relieving Casali in the seventh inning, he hit a home run on the second pitch he saw, a 94 mph (151 km/h) fastball from Duane Underwood Jr. of the Chicago Cubs.

[21] Pinch-hitting for leadoff hitter Shogo Akiyama in the first game of a doubleheader on September 14, Stephenson hit his first walk-off home run to win 3–1 against the Pittsburgh Pirates.

[26] The offseason departure of Curt Casali allowed Stephenseon to appear on the Reds' 2021 Opening Day roster alongside Tucker Barnhart.

[31] The Reds fell short of postseason contention, finishing seven games behind the St. Louis Cardinals in the Wild Card race, but Stephenson had a strong rookie season, batting .286 with 10 home runs and 45 RBI.

[33] Stephenson received two third-place votes in balloting for NL Rookie of the Year, an award which was won by Cincinnati teammate Jonathan India.

[34] The Reds traded Barnhart to the Detroit Tigers during the 2021–22 offseason, with general manager Nick Krall telling reporters that the move allowed Stephenson to become Cincinnati's everyday catcher in 2022.

[38] Stephenson was injured again on June 10, fracturing his right thumb on a foul tip off the bat of Jordan Luplow during Cincinnati's game against the Arizona Diamondbacks.

[43] This plan was abandoned by the end of May, with manager David Bell telling reporters that learning first base was "a lot to handle" for a young player like Stephenson.

[45] He also struggled defensively, particularly with pitch framing,[46] and young pitchers like Hunter Greene and Andrew Abbott preferred to work with Maile, which limited Stephenson's playing time towards the end of the season.