He became a resident envoy to Peru in 1935, accredited to several Latin American countries, and later served in Moscow, where he was involved in trade negotiations with the Soviet Union.

He was later appointed deputy state secretary for foreign affairs and played a significant role in the restoration of the Arvfurstens palats, the ministry's headquarters.

Assarsson remained active in business and cultural circles, joining the boards of several companies and supporting causes like flood relief efforts in Spain.

[2] His maternal grandfather was Bishop Wilhelm Flensburg [sv; da; de], and his brother was the Catholic priest David Assarsson.

In 1938, he became resident envoy in Mexico, with accreditations in Costa Rica, Cuba, Guatemala, El Salvador, Honduras, Nicaragua, and Panama.

Assarson became envoy to Russia in 1940 and was stationed in Moscow where he served as a delegate in Swedish-Soviet trade treaty negotiations that same year.

[3] During the early years of World War II, he experienced the conflict firsthand and participated in the diplomatic corps' evacuation to Kuybyshev.

[6] Assarsson himself was convinced that his expulsion was purely a retaliatory move in response to Sweden's refusal to release and repatriate V. Sidorenko, the head of Intourist in Stockholm, who had been exposed as the leader of a Soviet spy ring.

[8] When the Swedish government repeatedly refused to comply with Soviet demands, Assarsson and his military attaché were expelled from Moscow in December 1943.

Hjorth & Co.[11] He also served as a board member of the Tessin Society (Tessinsällskapet) until 1959, as well as AB Bacho and Contactor Co.[3] In November 1957, Assarsson, together with Rear Admiral Einar Blidberg, was among the 17 signatories of an appeal issued by a committee calling for Swedish aid to those affected by the devastating floods in Valencia, Spain, the previous month.

[7] Assarsson's literary tastes were primarily focused on classic literature, with Michel de Montaigne and Charles Augustin Sainte-Beuve among his favorite authors.

[7] In 1967, according to the Ordenskalender, he was reported to be the second most decorated Swede, with 29 distinctions, ranking just behind Envoy Ove Ramel [sv], who held 35.