The Athletics activated Mengden (elbow) from the 45-day injured list Wednesday.
Mengden was first placed on the 60-day IL in mid-February after requiring arthroscopic right elbow surgery, but he made a full recovery from the procedure while MLB was on hiatus for four months. After satisfying the Oakland coaching staff with his performance during summer camp, Mengden will open the season as the Athletics' fifth starter, taking the spot that was opened by A.J. Puk's (shoulder) move to the 10-day IL.