The film tells the story of neurosurgeon-turned-sorcerer Stephen Strange, played by Cumberbatch, 39, however, the 36-year-old "Midnight in Paris" actress' role is yet to be revealed, reported Entertainment Weekly.
"Doctor Strange" is directed by Scott Derrickson, of "Sinister" fame.
McAdams joins the cast which includes Tilda Swinton as Strange's mentor, the Ancient One, and Chiwetel Ejiofor as the villainous Baron Mordo.
The actress was recently seen in the romantic comedy "Aloha", the boxing drama "Southpaw" and the second season of HBO's "True Detective".
"Doctor Strange" is scheduled to release on November 4, 2016.