After passing additional examinations, he became an assessor at the higher court in Fulda in 1842 and at the higher court in Marburg in 1845. Pfeiffer's time in Marburg was particularly contentious, because the court expected him to help them perpetrate what in his eyes was an undue and unnecessary campaign against the Friends of the Light, a rationalist and humanist protestant sect then developing in Germany. His written decision regarding the group reflected his own liberal principles, and did little to endear him to his superiors on the court. In 1847 he became Amtmann in Felsberg and then state magistrate in Rotenburg an der Fulda. In addition to his legal positions, Pfeiffer also undertook duties a parliamentarian; from 1849 to 1850, he served in the , where he was the acknowledged legal expert for the Constitutional Party. In 1850, having sufficiently angered Ludwig Hassenpflug during his time in parliament, Pfeiffer was relegated to the position of superior court judge in Fulda. Later that year, Pfeiffer was elected to the (lower house) of the Erfurt Union Parliament, and from 26 March to 29 April of that year, served in the capacity of secretary. In 1851, following the Hessian constitutional crisis under Hassenpflug, Pfeiffer retired from all official duties for the Hessian state.
