Miller began the 2023 season with the Double-A Midland RockHounds, starting one game for the team before being moved up to Triple-A Las Vegas. After one start in Triple-A, on April 18, 2023, with 11 strikeouts over 5 perfect innings, the Athletics announced that Miller would be promoted to the major leagues for the first time.[9] Miller made his debut a day later, in a game against the Chicago Cubs. Miller pitched 41⁄3 innings, allowing two runs on four hits, a walk, and five strikeouts.[10] He also became the 10th starting pitcher since the debut of Statcast to throw 15 pitches above 100 miles per hour (160 km/h).[11] After a May 8 start against the Kansas City Royals,[12] it was reported that Miller was dealing with elbow tightness.[13] He was later diagnosed with a mild sprain of the ulnar collateral ligament in his throwing elbow and promptly shut down from throwing.[14] On June 30, he was transferred to the 60-day injured list.[15] On September 6, Miller was activated from the injured list, with manager Mark Kotsay noting that Miller would be slated for a relief role.[16]
In an effort to maintain Miller's health, the A's announced that Miller would be a reliever during the 2024 season.[17] Beginning the season as the team's closer, Miller earned his first career save in a 4–3 win at Texas on April 9.[18] Miller was named MLB Reliever of the Month for April 2024 after earning 8 saves in 8 opportunities and 28 total strikeouts.[19]
On July 7, 2024, Miller was named as a reserve player in the 2024 Major League Baseball All-Star Game.[20]
On September 26, 2024, Miller became the last pitcher to throw a pitch at the Oakland Coliseum in a major league game.[21]