The sitcom - now in its ninth series - is to celebrate its 100th episode in April.
Barrowman will play a dashing American medic who comes to Susan's rescue when she has her handbag snatched, while Haig will play a famous dentist who makes Ben jealous.
Barrowman said of his role: "It was great to be asked to guest star in My Family, especially as it was for such a special episode. I love the antics of the Harpers and my time spent on set was fantastic."
“Guest appearances from John Barrowman, David Haig and all the other great actors over the past nine series have been amazing," said BBC College of Comedy's Micheál Jacob.
"However, it’s the wonderful cast, talented writers and production team that have helped to secure My Family in the heart of the nation, and we hope that its success will continue long into the future.”
Comedian Julian Clary recently made a guest appearance in the My Family Christmas special.