At the age of thirteen, Ninidze auditioned for the part of Prince Edward in Shakespeare's Richard III at Rustaveli State Academic Theater in Tbilisi, which was directed by Robert Sturua and premiered in 1979. The production had great success, touring the UK three times, taking part in Edinburgh Festival, Glasgow Mayfest and The Roundhouse in London. From 1982 to 1985 he studied acting at Shota Rustaveli Theater and Georgia State Film University under the tutelage of Gizo Jordania, which led to him becoming a full-time member of the Shota Rustaveli Theatre Company and successful collaborations in many of Jordania's later productions, such as The Diary of Anne Frank in 1989, David Kldiashvili’s Step-Mother in 1989, which toured the UK in 1989–1990, and Hamlet in 1992. After Hamlet premiered in 1992, Civil War broke out in Georgia leading to his emigration, and he only returned to the theatre stage playing Shylock in The Merchant of Venice in 2003 (dir. Levan Tsuladze). He also took part in Georg Büchner’s Leonce and Lena (dir. Nana Kvaskhvadze) in 1991.