Seattle activated Wilhelmsen on Friday before opening a three-game series with Oakland. Wilhelmsen had been on the disabled list since April 11 after suffering a hyperextended right elbow.
Manager Lloyd McClendon had indicated a rehab assignment was likely for Wilhelmsen but after he threw a 25-pitch simulated game on Wednesday, the Mariners decided to activate him to the major league roster.
Wilhelmsen has made two appearances this season, giving up two runs in 2 2/3 innings.
Seattle optioned right-hander Dominic Leone to clear a roster spot for Wilhelmsen. Leone was 0-3 with a 5.40 ERA in eight appearances this season.