While attending the Actors' Workshop, Runacre was approached by fellow student (and future agent) Tom Busby, who was working as a runner for an American film production that was seeking fledgling English actresses to play opposite John Cassavetes in Husbands, a film to be shot the following year in London.

The young actress auditioned with Cassavetes, Ben Gazzara and Peter Falk, and was told six weeks later that she was being offered the part of Mary Tynan in the film.

Runacre left the cast after a year and starred in such films as Pier Paolo Pasolini's The Canterbury Tales, John Huston's The Mackintosh Man, Robert Fuest's The Final Programme, Michelangelo Antonioni's The Passenger, and Derek Jarman's Jubilee, in which she starred as Elizabeth I and "Bod".

She was active in the theatre as well as on British television, including an appearance as an art con artist in an episode of Lovejoy as well as playing Brenda Champion in the noted series Brideshead Revisited.

She also directed the Wireless Theatre Company's audio adaption of Frozen after its successful stage run, featuring the original cast.