Professionally, Mitrea started refereeing in the National Championship of Excellence in 2009 and made the step up to European rugby during the 2011–12 Pro12 - his first match being Connacht vs Scarlets on 10 September 2011. He also debuted at European level, refereeing during the 2011–12 European Challenge Cup before making the jump up to the Heineken Cup in 2013.