Fellow former soap star Samantha Womack will also be joining Harman on the show.
Harman and Womack will play Bradley and Tanya, a "sexy thirty-something couple" who bring some "down South mystique" to the Northern cul-de-sac.
"Bradley's a great character, a real wide boy who's clearly got something to hide. He and Tanya are definitely going to stir things up in Mount Pleasant but whether that's for the better or worse you'll have to wait and see," said Harman.
Samantha Womack added: "I'm thrilled to be joining the cast of Mount Pleasant, such a popular and well-loved series. I can't wait to get stuck into playing the part of Tanya who is sure to create some waves in The Close."
Mount Pleasant's third series will air in the summer on Sky Living.