In the first five games of August, McGehee is 2-for-16 (.125) with no extra-base hits or RBIs.
Donovan Solano got the nod at third base in the series finale at Pittsburgh.
"Casey has been grinding it out," manager Mike Redmond said. "It's been coming. I've been wanting to give him one, and today seemed like a perfect day."
McGehee, who was available off the bench if needed, appeared in 112 of Miami's first 113 games.
The only Marlin to appear in all 114 games is All-Star Giancarlo Stanton.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter