After finishing school in Preetz, Christiansen worked at Lufthansa from 1976 to 1982, then apprenticed in journalism at the Norddeutscher Rundfunk in Hamburg until 1985.

She ultimately became a regular presenter for the ARD's national TV news journal Tagesthemen, in rotation with Hanns Joachim Friedrichs (1987–1991) and Ulrich Wickert (1991–2004).

Along with prominent German political and celebrity guests, Christiansen interviewed international political and business leaders including Bill Gates (1999, 2002), James Baker (2000), Enrique Barón Crespo (2000), Wolfgang Schüssel (2000), Tony Blair (2002), Jean-Claude Juncker (2003), Hillary Clinton (2003), Donald H. Rumsfeld (2003), Bill Clinton (2004), Kofi Annan (2004) and Condoleezza Rice (2004).

[2] In addition, Christiansen moderated the TV election debates between Chancellor Gerhard Schröder and his challengers Edmund Stoiber (2002) and Angela Merkel (2005).

In 2015 she signed an open letter which the ONE Campaign had been collecting signatures for; the letter was addressed to Angela Merkel and Nkosazana Dlamini-Zuma, urging them to focus on women as they served as the head of the G7 in Germany and the African Union in South Africa respectively, which will start to set the priorities in development funding before a main UN summit in September 2015 that will establish new development goals for the generation.