 Thornton made his MLB debut on June 27, , with the Mariners in a game against the San Diego Padres, pitching brilliantly over 4 innings, only allowing 3 hits and striking out one batter.Throughout the season, the Mariners used Thornton for mostly middle reliever duty, except for one game when Thornton started and pitched 8 innings, allowing 4 runs on 8 hits and walking and striking out 7 batters.Thornton finished the 2004 season with a 1–2 record and 4.13 ERA, striking out 30 batters in 32.2 innings of work.