Alströmer began his diplomatic career in 1908 at the Swedish Ministry for Foreign Affairs, rising to first secretary by the end of that year.

During World War I, he served as acting head of the Ministry's Legal Division and later took on key international roles, including chargé d'affaires in London (1918–1919) and diplomatic postings in Paris and Kristiania (now Oslo).

In 1921, he was appointed resident minister and head of the personnel and administrative department at the Ministry, followed by positions as consul general in Shanghai and envoy to Switzerland, Vienna, and Budapest.

He represented Sweden at several international conferences, including those of the Red Cross, and participated in trade negotiations with Greece and Romania.

Later that year, in June, he joined the Swedish Forest Service (Domänstyrelsen), and by 1 December 1906, he was appointed valet de chambre.

From 18 December 1918 to 4 November 1919, he served as chargé d'affaires ad interim in London and was awarded the title of legation counsellor on 12 September 1919.

On 28 June 1922, he became envoy extraordinary and minister plenipotentiary to the Swiss Confederation, and from 27 September 1924, he simultaneously held postings in Vienna and Budapest.

[1] 3rd marriage: On 11 July 1936 in Vreta kloster socken, Östergötland County, to Ebba Jeanette Adine Peyron (born 14 January 1883 in Helsingborg).