Terletskyi was born in a village of Lozovyi Yar (near Yahotyn) in a family of priest.
He graduated Poltava Theological Seminary and later studied at a recently established Petrograd Psychoneurological Institute.
Terletskyi was elected to the Russian Constituent Assembly from Poltava Governorate as a member of the Ukrainian Party of Socialist Revolutionaries.
In 1918 he was involved in negotiations of the Treaty of Brest-Litovsk and later assassination of German field marshal Hermann von Eichhorn.
Terletskyi was one of founders of the Ukrainian party of left socialist-revolutionaries Borbysts (not to be confused with Borotbists).