Neokosmos Grigoriadis was born in 1879 in Constantinople, Ottoman Empire (today Istanbul, Turkey) to a Greek family.

He initially graduated from the Hellenic Army's NCO Academy but eventually he became a regular commissioned officer, and by the end of his military career he had achieved the rank of Major General.

[1] In 1920, after the royalist electoral victory, he was discharged from the Army, but he was recommissioned in 1922 by the 11 September 1922 Revolution, and participated in the Trial of the Six as "revolutionary commissioner" (public prosecutor).

Grigoriadis was then involved in politics, being elected senator for Edessa (renamed from Vodena) for Venizelos's Liberal Party (1929-1935).

After the German invasion of Greece and the Nazi occupation, Grigoriadis was an active member of the left-wing National Liberation Front (EAM), culminating in his role as Chairman of the "National Council" that EAM created in 1944 in order to supervise the drafting of a new constitution for post-liberation Greece.