Teodor, born Calmăşul, changed the family name to the Greek form Callimachi (Καλλιμάχης).

Ioan's older brother, Gavriil Callimachi (1689–1786) was a monk at Putna Monastery.

He knew Latin, Turkish, Italian, Greek and French.

Callimachi served in the administrations of John Mavrocordatos and of Grigore II Ghica.

He was Grand Dragoman at the Ottoman Porte in Istanbul where, over the course of his sixteen years of service, he was recognized for his diplomatic ability.