Born Kiel, Walther studied literature, history and philosophy at the Goethe University Frankfurt from 1980 to 1986 and completed an editorial training course at a daily newspaper.
In 1992 he received his doctorate in Frankfurt am Main with a thesis on the historian and politician Barthold Niebuhr supervised by Ulrich Muhlack.
In the summer of 1993 Walther was awarded the Friedrich Sperl Prize of the Association of Friends and Sponsors of the Johann Wolfgang Goethe University Frankfurt.
In 1997 he also completed his habilitation in Frankfurt with a thesis on the Fulda prince abbot Balthasar von Dernbach.
Since 2002, he has been teaching in the succession of Hartwig Brandt as professor of early modern history at the Bergische Universität Wuppertal.