McKay had his contract selected by the Mariners on Saturday, Ryan Divish of The Seattle Times reports.
McKay spent part of spring training in major-league camp before being reassigned in mid-March. The young right-hander had a 2.49 ERA, 1.13 WHIP and 71:21 K:BB across 50.2 innings for Double-A Arkansas last season and figures to start out in a low-leverage bullpen role.