Mills was recalled by the Mariners on Thursday, Ryan Divish of The Seattle Times reports.
Mills began the season at Triple-A Tacoma and logged a 1.94 ERA and 0.77 WHIP in 9.1 innings during six relief appearances. In spite of his minor-league success early in the year, he's likely to be utilized mainly as a low-leverage reliever after he posted a 9.95 ERA and 2.05 WHIP in 12.2 major-league innings last season.