He was sent to Norway in 1773 as a first lieutenant of Søndenfjeldske regiment, and rose to the rank of captain and company commander in 1779 and major in 1788.

In 1789 he was appointed generalkrigskommissær, the officer in charge of national conscription, and in 1802 became the director of the War Academy (Krigsskolen).

He spent the years 1808–1810 in Denmark as head of the war commissariate, but retained nonetheless all of his Norwegian posts.

Haxthausen had a major influence on Prince Christian Frederick as viceroy (stattholder) of Norway from 1813, joined the interim government of Christian Frederick in March 1814, and on 19 May 1814 he became Minister of Finance in the first cabinet of independent Norway.

During the Swedish campaign against Norway in 1814 he served as a lieutenant general, but was wrongly accused of being a traitor, and on 19 August, 5 days after the Convention of Moss, his house and garden was attacked by a mob.