In 1917, he joined the Ministry for Foreign Affairs, where he served in various roles including attaché, vice-consul, and first legation secretary.
[3] During his studies, he served on the board of the national organization of the Young Swedish Association (Ungsvenska förbundet)[4] and was treasurer of the Stockholm University College Student Union.
[5] Kastengren worked as an assistant lawyer at a law firm in Stockholm from 1916 to 1917, before being employed as an attaché at the Ministry for Foreign Affairs in 1917.
After spending 10 years in the United States, he was appointed Envoy to Lima in 1945, with dual accreditation to La Paz and Quito.
[1] On 19 July 1946, he presented his credentials to Bolivia's President Gualberto Villarroel, two days before the latter was assassinated in the presidential palace.
[1][11] They had one son named Bengt Kastengren (1924–2014), CEO of the insurance company Återförsäkrings AB Atlas.