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.

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.

[1] In February 1814 he was summoned by another Danish Prince, Christian Frederick, to participate in the Meeting of Notables,[2] which set the principles for a later Norwegian Constituent Assembly.

In principle it was a copy of Christian August's actions in 1808; specifically Sejersted did not wish to invade Sweden, only route Swedish forces from Norway if necessary.

[1] In January 1814 the Treaty of Kiel was signed, giving Sweden (as a victorious country in the Napoleonic Wars) control over Norway.

[1] In sum Sejersted appealed fruitlessly to Christian Frederick to enact a truly forceful defence of Norway,[2] and in the latter phase of the conflict he stood without real influence.