Jahn was born in Sandlack, East Prussia (today Sędławki, Poland), where she grew up. From 1934 to 1937 she attended school in Berlin and began her studies in chemistry at the University of Munich in 1940. There Jahn became a close friend of Hans Conrad Leipelt and a member of the White Rose resistance group. After Hans and Sophie Scholl and Christoph Probst had been imprisoned she continued to publish the Scholl leaflets and collected money to aid the widow of Kurt Huber. In October 1943 she was also arrested by the Gestapo and sentenced to 12 years' imprisonment by the Volksgerichtshof in 1944.
After her liberation she studied medicine at the University of Tübingen and worked as a physician in Bad Tölz. In 1987 she was a founding member of the White Rose Foundation and member of the executive board until 2002.
She died on 22 June 2010 in Bad Tölz.