Horn was elected lantmarskalk, for the Caps Party ("Mössorna") at the Riksdag of 1720, and contributed, on the resignation of Ulrika Eleonora, to the election of Frederick of Hesse as king of Sweden, whose first act was to restore to him to the Privy Council and as President of its Chancellery, in effect as Prime Minister. For the next eighteen years he so absolutely controlled both the foreign and the domestic affairs of Sweden that the period between 1720 and 1738 has well been called the Horn period.