Her film appearances include Braveheart (1995), The Land Girls (1998), Dangerous Beauty (1998), Dancing at Lughnasa (1998), Spy Game (2001), and 28 Weeks Later (2007).

[3] Her mother died of lupus when McCormack was six years old and her steelworker father subsequently raised her and her brother Stephen.

"[4] After Braveheart, McCormack starred alongside Anna Friel and Rachel Weisz in David Leland's The Land Girls and had lead roles in Nils Gaup's Northstar and Marshall Herskovitz's Dangerous Beauty.

[9] In 2008, she performed the role of Nora in A Doll's House,[10] directed by Peter Hall at the Theatre Royal, Bath, and also the role of Isabel Archer in a stage adaptation of The Portrait of a Lady,[11] both of which commenced their runs in July 2008, ending in August, before transferring to the Rose Theatre in Kingston later that year.

In 2012, she starred as Juana Inés de la Cruz in the Royal Shakespeare Company's production of Helen Edmundson's play The Heresy of Love.