[1] Born in Romania, Mitrea moved to Treviso, Italy in 1999 after he finished high school, where he learnt Italian, Spanish, French and English on top of his first Romanian language.

While in Galați, Marius played for his local age-grade rugby team and continued with the sport once moved to Italy.

[1] 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 made his international debut in 2010, where he refereed Belgium vs Canada in Brussels.

In September 2015, he went to the 2015 Rugby World Cup as an assistant referee.