He was a member of the Provisional Assembly of the Autonomous Estonian Governorate (1917–19) and the Constituent Assembly of the newly established Republic of Estonia (1919–20); he also served as a minister in Jaan Raamot's Provincial Government (Estonian: Eesti Maavalitsus) in 1917, in the Second and Third Provisional Governments (1918–19), and in Otto Strandman's first Cabinet and Jaan Tõnisson's first Cabinet.
In later life, he worked as an academic at the University of Tartu and as an adviser to a number of national banks.
Köstner was born on 26 March 1889 in Luunja Parish in Kreis Dorpat, Governorate of Livonia,[1] the son of Jaak Kostner and Ann, née Jermakoff.
In 1919, he was elected to the Constituent Assembly of the newly established Republic of Estonia as a member of the Estonian Social Democratic Workers' Party, but he stepped down on 3 February 1920, to be replaced by Johan Epner.
[3] Köstner's parliamentary career ended there, but he also held a range of government offices.