In 1851, he was elected president of the chamber and, in the same year, minister of justice, the first Catholic who had held so high an office in Hanover. As minister, he carried through an important judicial reform—which had been prepared by his predecessor—but had to retire from office because he was opposed to the reactionary measures for restoring the influence and privileges of the nobility. Though he was always an enemy to liberalism, his natural independence of character prevented him from acquiescing in the reactionary measures of the king. In 1862, he again was appointed minister, but with others of his colleagues, he resigned when the king refused his assent to a measure for extending the franchise. Windthorst took no part in the critical Austro-Prussian War; contrary to the opinion of many of his friends, after the annexation of Hanover by Prussia, he accepted the "fait accompli", took the oath of allegiance, and was elected a member both of the Prussian parliament and of the North German diet.