During World War I, he served as a general staff officer in the 1st Army Division, and in 1918, he played a significant role in the Finnish Civil War, becoming captain and lieutenant colonel in the Finnish Army and participating in key battles.

Returning to Sweden, Douglas continued his military career, holding various positions such as chief of staff of the 5th Army Division and commanding officer of the Norrland Dragoon Regiment.

In addition to his military duties, Douglas was a member of the Stockholm City Council and held roles as aide-de-camp and chief aide-de-camp to King Gustaf V. Apart from his military career, Douglas was also a prolific writer, producing biographical literature, including works on Marshal Carl Gustaf Emil Mannerheim and his ancestor, Swedish Field Marshal Robert Douglas, Count of Skenninge.

Back in Sweden, Douglas became ryttmästare and squadron commander in the Life Regiment Dragoons in 1919 and taught ground warfare at the Royal Swedish Naval Staff College on and off between 1919 and 1929.

He served as chief of staff of the 5th Army Division (V. arméfördelningen) from 1923 to 1927 when he became major in the Life Regiment Dragoons.

In 1942 he was appointed Inspector of the Swedish Army and the year after he was chairman of the Officer Training Investigation (Officersutbildningsutredningen).

The Meyerhöffer affair highlighted internal conflicts within the Swedish military and its relations with Nazi Germany.

[4] Douglas lived in Villa Parkudden in Djurgården,[9] and also owned Stjärnorp Castle in Östergötland County from 1947.

[13] His eldest son, the Swedish Ambassador to Brazil Carl Douglas, also died in a car accident, just six months later.