In 1868 Gneist became a member of the North German parliament, and acted as a member of the commission for organizing the federal army, and also of that for the settlement of controversial ecclesiastical questions. On the establishment of German unity his mandate was renewed for the Reichstag, and there he served as an active and prominent member of the National Liberal party, until 1884. In the Kulturkampf he sided with the government against the attacks of the Clericals, whom he bitterly denounced, and whose implacable enemy he ever showed himself. In 1879, together with his colleague, Hänel, he violently attacked the motion for the prosecution of certain socialist members, which as a result of the vigor of his opposition was almost unanimously rejected. He was parliamentary reporter for the committees on all great financial and administrative questions, and his profound acquaintance with constitutional law caused his advice to be frequently sought, not only in his own but also in other countries. In Prussia he greatly influenced legislation, the reform of the judicial and penal systems and the new constitution of the Evangelical Church being largely his work. In 1875, he was appointed a member of the supreme administrative court ("Oberverwaltungsgericht") of Prussia, but only held office for two years.