He has an immense list of acting roles to his name, including many UK television dramas. He has appeared in Doctor Who, Rebus and The Bill. He is known for his supporting role in James Cameron's Titanic. He starred alongside Susan Lynch and Ronald Pickup in a 1997 TV adaptation of Ivanhoe.
Donachie, Rose Leslie, Iain Glen, Simon Lowe, and James Faulkner, who each appear in a different season of Downton, and Mark Addy, who appears in the film, all star in the series Game of Thrones. Donachie portrayed Ser Rodrik Cassel in Game of Thrones.