Benoit, 40, was 1-4 with two saves and a 4.07 ERA in 44 appearances. He joins his eighth team in his 16 seasons in the majors.
McGarry had a 1.34 ERA in 31 games for Class A Bradenton this season. He was selected in the eighth round of the 2015 draft from Florida Atlantic.
Philadelphia also traded veteran reliever Pat Neshek, starter Jeremy Hellickson and infielder/outfielder Howie Kendrick in the past week. Pittsburgh traded lefty reliever Tony Watson to the Dodgers earlier Monday.