'Paradise' star Hunter impressed director Snyder who is working on the sequel to 2013 blockbuster 'Man of Steel' and although the role is yet to be named he claims it was written "specifically" for her.
Snyder said: "Holly has always been one of my favourite actresses. She has immense talent and is always captivating on screen. I had an opportunity to meet her a while back and knew instantly that I had to work with her, so as we began writing the script I made sure to create a role specifically for her."
Mulvey will appear alongside Henry Cavill who will reprise his role as Superman and Ben Affleck as Batman, and Snyder was thrilled to announce they'll be teaming up once again after filming '300: Rise of an Empire' together.
He explained: "I just had the good fortune to work with Callan on '300: Rise of an Empire' and was very impressed with his incredible talent.
"He's a fantastic actor and I'm looking forward to having the chance to work with him again."
Warner Bros has also cast Tao Okamoto who starred in 'The Wolverine' in the forthcoming film.
Snyder gushed: "And, quite simply, Tao is a striking presence whose beauty is aptly rivaled by her amazing abilities as an actress. I'm really excited to have her joining us on this adventure."
The film is currently being written by Chris Terrio, from a screenplay by David S. Goyer.
Charles Roven and Deborah Snyder are producing, with Benjamin Melniker, Michael E. Uslan, Wesley Coller, Goyer and Geoff Johns serving as executive producers.
The film is tipped for a May 6, 2016, release.