Poska was born the fifth of 12 children of a Eastern Orthodox parish schoolmaster and received his secondary education at the church's school in Riga.

15 November] 1917 the Maapäev refused to recognize the new Bolshevik rule and proclaimed itself the supreme legal authority of Estonia.

The Republic of Estonia declared independence on 24 February 1918, only to be occupied by the German Empire until the end of World War I in November 1918.

He worked in Western Europe for gaining diplomatic recognition to Estonia and participated in Paris Peace Conference.

He led the peace talks with Soviet Russia and achieved Treaty of Tartu which was signed on 2 February 1920.