Soldier and diplomat
After completing his studies at Åbo, he entered the Swedish army and served for several years in the Netherlands, in Hungary under Prince Eugene, and in Flanders under Waldeck (1690-1695). He stood high in the favour of the young Charles XII of Sweden and was one of his foremost generals in the earlier part of the Great Northern War. In 1704 he was entrusted with his first diplomatic mission, the deposition of Augustus II of Poland and the election of Stanislaus I of Poland, a mission which he accomplished with distinguished ability but absolute unscrupulousness. Shortly afterwards he was besieged by Augustus in Warsaw and compelled to surrender.
In 1705 he was made a Privy Councillor, in 1706 a count and in 1707 governor of Charles XII's nephew, the young duke Charles Frederick of Holstein-Gottorp. In 1710 he succeeded Nils Gyldenstolpe as President of the Privy Council Chancellery. Transferred to the central point of the administration, he had ample opportunity of regarding with other eyes the situation of the kingdom, and in consequence of his remonstrances he fell rapidly in the favour of Charles XII. Both in 1710 and 1713 Horn was in favour of summoning the estates, but when in 1714 the diet adopted an anti-monarchical attitude, he gravely warned and ultimately dissolved it. In Charles XII's later years Horn had little to do with the administration. After the death of Charles XII, in 1718 it was Horn who persuaded the princess Ulrika Eleonora of Sweden to relinquish her hereditary claims and submit to be elected queen of Sweden. He protested against the queen's autocratic behaviour, and resigned from the Privy Council.
He was elected lantmarskalk 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 asPrime 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. His services to his country were indeed inestimable. His strong hand kept the inevitable strife of the parliamentary factions within due limits, and it' was entirely owing to his provident care that Sweden so rapidly recovered from the wretched condition in which the wars of Charles XII had plunged her. In his foreign policy Horn was extremely wary and cautious, yet without compromising either the independence or the self-respect of his country. He was, however, the promoter of a new principle of administration which in later days proved very dangerous to Sweden under ministers less capable than he was. This was to increase the influence of the diet and its secret committees in the solution of purely diplomatic questions, which should have been left entirely to the executive, thus weakening the central government and at the same time facilitating the interference of foreign Powers in Sweden's domestic affairs. Not till 1731 was there any appearance of opposition in the diet to Horn's "system"; but Horn, piqued by the growing coolness of the king, the same year offered his resignation, which was not accepted.
In 1734, however, the opposition was bold enough to denounce his neutrality on the occasion of the War of the Polish Succession, when Stanislaus I of Poland again appeared upon the scene as a candidate for the Polish throne; but Horn was still strong enough to prevent a rupture with Russia. Henceforth he was bitterly but unjustly accused of want of patriotism, and in 1738 was compelled at last to retire before the impetuous onslaught of the triumphant young Hat party. For the rest of his life he lived in retirement at his estate at Ekebyholm, where he died on April 17, 1742. Horn in many respects greatly resembled his contemporary Robert Walpole. The peculiar situation of Sweden, and the circumstances of his time, made his policy necessarily opportunist, but it was an opportunism based on excellent common sense.