Pirkis started his acting in school plays while attending Eton, though he never took them seriously. Although unsure what he wanted to do with his life, Pirkis was cast for his first professional role in 2003, when he appeared in the successful film, Master and Commander: The Far Side of the World. The hire occurred after production staff visited Eton to hire boys for the ship's teenage crew. Pirkis attended a workshop and auditions as "a joke", ultimately securing the part of the 13-year-old, eventually one-armed midshipman Lord Blakeney of HMS Surprise. Director Peter Weir thought Pirkis "stood out very early on, but not to the extent that you see in the film. That was a complete surprise." According to Pirkis, he was at first "nervous" about acting with Russell Crowe, but changed his mind when he realised that Crowe was "a pretty normal guy."