The four-hour drama, based on the novel by Stephen King, stars Pierce Brosnan as a writer who is recovering from the death of his wife.
When he begins to have nightmares about their lakehouse in Maine, he moves there in an attempt to conquer his fears.
Priestley has now joined the cast as the literary agent of Brosnan's character, Entertainment Weekly reports.
Elsewhere, William Schallert has signed up to play the father-in-law of a young woman (Melissa George) going through a custody battle.
Priestley has previously appeared in shows such as Beverly Hills, 90210, Tru Calling and Call Me Fitz. More recently, he signed up for a guest role in Psych and joined the cast of Haven.
Meanwhile, Schallert appeared in The Patty Duke Show and has had guest roles in programmes like True Blood, Medium and Desperate Housewives.
Bag of Bones is expected to air on A&E later this year.
> Stephen King's 'Under The Dome' in works at Showtime