When in 1738 the Hats came into power the younger Höpken obtained a seat in the secret committee of the diet, and during the Finnish war of 1741-42 was one of the two commissioners appointed to negotiate with Russia. During the diet of 1746-1747 Höpken's influence was of the greatest importance. It was chiefly through his efforts that the estates issued a "national declaration" protesting against the arrogant attitude of the Russian ambassador, who attempted to dominate the crown prince Adolphus Frederick and the government. This spirited policy restored the waning prestige of the Hat party and firmly established their anti-Muscovite system.
In 1746 Hopken was created a senator. In 1751 he succeeded Carl Gustaf Tessin as president of the royal chancellery, and controlled the foreign policy of Sweden for the next nine years. On the outbreak of the Seven Years' War, he contracted an armed neutrality treaty with Denmark (1756); but in the following year acceded to the league against Frederick II of Prussia.
During the crisis of 1760-1762, when the Hats were at last compelled to give an account of their stewardship, Höpken was sacrificed to party exigencies and retired from the senate as well as from the premiership. On June 22, 1762, however, he was created a count. After the revolution of 1772 he re-entered the senate at the particular request of Gustavus III, but no longer exercised any political influence. His caustic criticism of many of the royal measures, moreover, gave great offence, and in 1780 he retired into private life.
Höpken was a distinguished author. The noble style of his biographies and orations has earned for him the title of the Swedish Tacitus. He helped to found the Royal Swedish Academy of Sciences, and when Gustavus III in 1786 established the Swedish Academy, he gave Höpken the first place in it.
See L. G. de Geer, Minne af Grefve A. J. von Höpken (Stockholm, 1882); Carl Silfverstolpe, Grefve Höpkens Skrifter (Stockholm, 1890-1893).