The 49-year-old actress's character Waller is described as a former congressional aide and government agent who oversees Suicide Squad in the comics, reported Ace Showbiz.
Previously, it was reported that Oprah Winfrey and Octavia Spencer were also circling the role alongside the 'How to Get Away with Murder' star.
'Suicide Squad' tells the story of a group of DC comics super villains who are sent to dangerous missions by the government to redeem their past.
Earlier this month, Warner Bros officially announced the cast which includes Jared Leto (The Joker), Will Smith (Deadshot), Margot Robbie (Harley Quinn), Tom Hardy (Rick Flag), Jai Courtney (Boomerang) and Cara Delevingne (Enchantress).