After a careful education and a long residence abroad, he began his diplomatic career at the great peace congress of Osnabrück. During his stay in Germany he made the acquaintance of the count palatine, Charles Gustavus, afterwards Charles X of Sweden, whose confidence he completely won. Two years after the king’s accession in 1654, Oxenstierna was sent to represent Sweden at the Kreistag of Lower Saxony. In 1655 he accompanied Charles to Poland and was made governor of the conquered provinces of Kulm, Kujavia, Masovia and Great Poland. The firmness and humanity which he displayed in this new capacity won the affectionate gratitude of the inhabitants, and induced the German portion of them, notably the city of Thorn, to side with the Swedes against the Poles. During Charles’s absence in Denmark in 1657, Oxenstierna in the most desperate circumstances, tenaciously defended Thorn for ten months, and the terms of capitulation ultimately obtained by him were so advantageous that they were made the basis of the subsequent peace negotiations at Oliva, between Poland and Sweden, when Oxenstierna was one of the chief plenipotentiaries of the Swedish regency. During the domination of Magnus de la Gardie he played but a subordinate part in affairs. From 1662 to 1666 he was governor-general of Livonia. In 1674 he was sent to Vienna to try and prevent the threatened outbreak of war between France and the empire. The connexions which he formed and the sympathies which he won here had a considerable influence on his future career, and resulted in his appointment as one of the Swedish envoys to the congress of Nijmegen (1676). His appointment was generally regarded as an approximation on the part of Sweden to Austria and the Netherlands. During the congress he laboured assiduously in an anti-French direction; a well-justified distrust of France was, indeed, henceforth the keynote of his policy, a policy diametrically opposed to Sweden’s former system.
In 1680 Charles XI of Sweden entrusted him absolutely with the conduct of foreign affairs, on the sole condition that peace was to be preserved, an office which he held for the next seventeen years to the very great advantage of Sweden. His leading political principles were friendship with the maritime powers England and the United Provinces, and the emperor, and a close anti-Danish alliance with the house of Holstein. Charles XI appointed Oxenstierna one of the regents during the minority of Charles XII of Sweden. The martial proclivities of the new king filled the prudent old Chancellor with alarm and anxiety. His protests were frequent and energetic, and he advised Charles in vain to accept the terms of peace offered by the first anti-Swedish coalition. Oxenstierna has been described as “a shrewd and subtle little man, of gentle disposition, but remarkable for his firmness and tenacity of character.”