 Detwiler began the year with the York Revolution, where he went 3-1 with a 2.70 ERA over 30.0 innings.The Seattle Mariners purchased his contract on June 1, 2018 and assigned him to the Triple-A Tacoma Rainiers.On August 21, Detwiler was called up to the Mariners' roster after going 2-5 with a 5.15 ERA in 14 games for Tacoma.