Plassnik is from the Austrian province of Carinthia and grew up in a social democratic family. 1971 to 1972 she was exchange student at the highschool in Foxcroft, Virginia, USA. She received a law degree in 1977 and worked thereafter in different positions in the Austrian Ministery for Foreign Affairs until 1997. Until that time she was was considered to be politically neutral or even social-democratic. July 1 1997 she was chosen by Wolfgang Schüssel, who was Austrian vice-chancellor at that time, to become his cabinet chief. In 2000 she refused an offer to work for the Council of Europe in order to stay in Austria and help Wolfgang Schüssel when he became Austrian Chancellor in a coalition government with the Austrian Freedom Party. She is considered to be a close friend and confidante to Schüssel. She remained cabinet chief in Schüssel's government until January 15 2004, then she served as Austria's ambassador to Switzerland.

She was appointed foreign minister on October 18 2004, and sworn in on October 20 succeeding Benita Ferrero-Waldner, who resigned to become a European commissioner. She joined the Austrian People's Party not until this offer to become foreign minister.

Plassnik has been married twice. Her first marriage was to Georg Posch , a social democrat politician. Her second marriage was to Swiss diplomat Gerard Stoudman .

|- | width="30%" align="center" | Preceded by:
Benita Ferrero-Waldner | width="40%" align="center" | Foreign Minister of Austria
since 2004 | width="30%" align="center" | Succeeded by:
--- |- |}