The couple tied the knot in September 2008.
“Rob and I are so excited it’s stupid,” Olson said in a statement to People.com. “The last couple years have been pretty amazing for us and just when we thought things couldn’t get any better, the greatest little thing in the world happened!
“We’re totally beside ourselves,” Olson added.
In addition to playing Mac, McElhenney is a writer-executive producer on the FX comedy. He also recently reprised his role of Aldo on ‘Lost.’
‘It’s Always Sunny’ returns for its sixth season in September.