Immediately after studying she was employed in Ljubljana City Theater. She was a member of this theater for ten years (1969-79), then she started working in the Ljubljana Drama, where she returned after a short period of creative work at the Slovene Youth Theater (1985-1988) and remained as a member of the ensemble until retirement. She performed in numerous roles in the various theater, film and television productions, and also in many radio-dramas.
She has created many character and supporting roles of emotionally complex women. In very different roles, she presents the same suggestive image from classical and contemporary drama. She was rewarded at the Academy in 1970, with the Student Prešeren Prize for Claire's role in Genet's Maids. In the same year she received Borštnik's prize for the role of Caetana in Hieng's Conqueror (performed by the City Theater of Ljubljana), which received the Sterija Award in the following year. A number of awards followed, at theater and film festivals, in the former Yugoslavia.
In the explanation of the Borštnikov ring award in 1999, Jernej Novak wrote that "Milena Zupančič is a born actress: she is open, perceptive, able to feel the contradictions of life within her, understand them and giving them the image of simultaneous struggle." Vasja Predan, in the actress's portrait, adds that her role "is always an imaginary, mature fetus and a very happy combination of talent, knowledge, extraordinary intuitive sensibility, intelligent understanding and experiential omnipotence: especially with an incredible feeling of tragicomic duality of split, often on the edge the grotesque and the tragedy of the stumbling or shaking modern women."
In 1993 she received the Prešeren Prize for Lifetime Achievement where she was described as an actress of a stunning acting range, which interprets the diversity of the roles "by performing a masterly parsing of the role, by practically playing contradictory psychological states, with picturesque dialogue amplitudes, with precisely measured use of movements, and brilliant voice interpretation ".
Her acting opus in the Slovene and Yugoslav films is by no means negligible. She created a characters that are indelibly recorded in the memory of many spectators: Meto in the film Flowers in the Autumn (1973) by Matjaž Klopčič; under his directorial direction, she also played student in the film Oxygen (1970), and then became Klopčič's long-time muse, and consequently stared in his movies Dediščina, The Search, My Dad, the Socialist Kulak and The Widowhood of Karolina Žašler. Among many feature films, she also starred as Tončka in the film Idealist by Igor Pretnar, and Lenka in Christophoros by Andrej Mlakar.