Käßmann earned her Ph.D. under Konrad Raiser, at the Ruhr University Bochum, with a thesis on the topic "Poverty and Wealth as an Inquiry into the Unity of the Church". In 1990, she was assigned to the Evangelical Church's volunteer service, and from 1992 to 1994, she was director of studies at the Evangelical Academy at Hofgeismar. Between 1994 and 1999, she was General Secretary of the "Deutscher Evangelischer Kirchentag" (German Protestant Church Congress). In 1999, she was elected bishop of the Evangelical Lutheran Church of Hanover; she was the first woman to hold this office. In 2006 she underwent a breast cancer operation.