The series, which is expected to take a "fresh look" at Arthurian legend, begins filming in Ireland this week.
Deadline reports that Forlani, who recently dropped out of Matt LeBlanc's show Episodes, will play Arthur's mother Queen Igraine. Meanwhile, Mooney will star in the programme as Arthur's brother Kay.
The cast of Camelot also includes Joseph Fiennes, Eva Green, Tamsin Egerton and Jamie Campbell Bower.
The show is expected to premiere on Starz early next year.