Mitchell's contract was selected by the Marlins on Thursday, Jordan McPherson of the Miami Herald reports.
Mitchell was let go by the Phillies in late July after he began the season at Triple-A Lehigh Valley, but he joined Miami on a minor-league pact in early August. The right-hander generated a 3.00 ERA and 1.33 WHIP in 12 innings across five appearances (one start) at Triple-A Jacksonville and will be available to make his 2021 major-league debut after right-hander Luis Madero was designated for assignment Thursday.