Born in affluent noble family, Kosynskyi received his secondary education from the Novhorod-Siverskyi gymnasium which finished in 1883.
In 1907 Kosynskyi was brought to criminal responsibility (abetment in civil disorder) and fired from the position of dean, but later was acquitted.
During the times of Russian Provisional Government Kosynskyi participated in the Central Land Committee and the State Economic Conference.
Since August 1917 Kosynskyi was a member of the Ukrainian Central Council from Constitutional Democratic Party as a representative of Russian minority.
In 1919 he was official authorized to return to his work in the Ukrainian Academy of Sciences and the Kyiv Polytechnic Institute.