He was posted as an attaché in New York City and Paris before taking on assignments in Tokyo and Beijing, where he acted as chargé d'affaires ad interim in 1912.
Over the years, he held key diplomatic positions in Shanghai, Washington, D.C., Copenhagen, Berlin, and London, often serving as chargé d'affaires ad interim.
During World War II, he remained in Romania, safeguarding the interests of multiple nations despite limited communication with Sweden.
He was the son of Carl Reuterswärd [sv] (1853–1932) and his wife, Baroness Rosa Elisabet Fransiska von Ungern-Sternberg (1856–1938).
His father was a lieutenant colonel in the Life Regiment Hussar Corps, while his mother, a member of the Ungern-Sternberg noble family and born in Heidelberg, Germany, became a lady-in-waiting to Queen Victoria of Baden in 1908.
Returning to Sweden, he was appointed second secretary at the Ministry for Foreign Affairs on 31 December 1912, and later served as chargé d'affaires ad interim from 11 October 1913 to 5 February 1914.
[3] His international postings continued with his appointment as first legation secretary in Copenhagen on 22 July 1918, where he also served as chargé d'affaires ad interim at various times in 1918 and 1919.
His career continued to rise, and on 13 June 1928, he was appointed envoy extraordinary and minister plenipotentiary in Reval (Tallinn), Riga, and Kovno (Kaunas).