He was born in
Flå in
Søndre Trondhjem county, as a son of Lieutenant Colonel Jens Fredrik Svane Sejersted and his wife Dorothea Catharina Klingenberg. He studied at the
University of Copenhagen from 1777 to 1781. In 1788 he served as an aide-de-camp of General Moltke during the
Theater War, when
Denmark-Norway attacked
Sweden. Sejersted remained in the military, and was promoted to second lieutenant in 1781 and premier lieutenant in 1789. In 1794, he joined the Dano-Norwegian General Staff, from 1795 as captain. He was promoted to major in October 1807, and at the same time stationed in
Norway. In early 1808 he assisted the Danish Prince
Christian August of Augustenborg in his campaigns in Aurskog-Høland, as a part of the
Dano-Swedish War. He would remain on Christian August's staff until the war's end in 1809. He was promoted to lieutenant colonel in August 1808 and colonel later that year. As Norway declared independence in
May the same year, Sejersted was appointed head of the newly created General Staff on 25 May 1814. He was also promoted to major general. In the same month Sejersted received a direct order to create a more offensive strategic plan. He did so, however the plan was never actually followed. In reality, then, Norway came to follow a defensive strategy. However, there were some important discrepancies between Sejersted's politically influenced plan and the battleground. Thus, Norwegian forces experienced a retreat from
Rakkestad near the Swedish border early in the conflict. Sejersted then crafted a new plan, but this was abandoned as Christian Frederick, in May crowned as King and thus
commander-in-chief, abandoned a Norwegian stronghold at
Langnes. A third defensive plan followed, but again the King acted against it to achieve a short-term goal. ==Later life==