When Marcus Wallenburg Sr. asked Cassell to recommend a Swede to join the Secretariat of the League of Nations, he suggested Per Jacobsson.

In April 1920 he became an international civil servant, by joining the League of Nations' Secretariat, and specifically what a few years later would be known as the Economic and Financial Organization.

[2] Piet Clement, a financial historian, wrote that Per Jacobsson's work for the committee "left a deep imprint on his further career...because it taught him to quickly analyze a multitude of qualitative and quantitative data at an aggregated and comparative level, and to formulate conclusions and recommendations in a nevertheless pertinent and precise style.

This was one element in the League's efforts to limit spending on armaments and reduce the risk of a major armed conflict recurring.

He continued his work on this after leaving the League as an outside expert, contributing to a report prepared for the World Disarmament Conference which first met in February 1932.

(After leaving the League, he returned to Sweden and worked from January 1929 to July 1930 at the Swedish Economic Defence Commission which was studying preparation for the eventuality of another war, and then from July 1930 to September 1931 as Economic Adviser to a Swedish industrial company, Kreuger and Toll.

In December 1956, he was appointed as the managing director of the International Monetary Fund (IMF), a position he held until his death on 5 May 1963.

Jacobsson's daughter Moyra, an artist, married Roger Bannister, the British Olympic athlete and neurologist who was the first person to run the four-minute mile.

The Per Jacobsson Foundation was established in 1964 to honor his memory and sponsors prestigious lectures, many of which have been given on the occasion of the annual meetings of the IMF and World Bank in Washington DC.