Guinan will play the father of Mike (Billy Gardell), who abandoned his family years ago, but reappears before Mike and Molly (Melissa McCarthy) tie the knot.
Guinan has been seen on Broadway in August: Osage County and The Grapes of Wrath. His television credits include "That 70s Show," "Boss," "Grey's Anatomy," "The Practice" and "Star Trek: Enterprise."
"Mike & Molly" airs Monday nights at 9:30 PM ET on CBS.