 Thornton had a shaky start to his career in Triple-A ball, starting 2 games and posting an 0–2 record and an 8.00 ERA.Thornton stayed with the Tacoma Rainiers in and posted a 7–5 record, along with a 5.20 ERA and 74 strikeouts in 83 innings pitched.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.