According to The Times, the actress - who rose to fame last year following her highly-acclaimed performance in An Education - beat 5,000 other candidates for the role of Lisbeth in the forthcoming film adaptation.
"Now she wants to do something bigger and bolder, without becoming a comic book heroine. This role will make her a household name," an insider revealed.
Meanwhile, the Oscar nominee explained last month: "I am obsessed with those books. I would love to do them. I am not going to lie about that. I would love to play Lisbeth Salander."
Mulligan could be paid up to £10m for the three films covering Stieg Larsson's trilogy, which has sold 1m copies in Britain and 25m worldwide.