He was born Ieronymos Kotsonis (Ιερώνυμος Κοτσώνης) to a poor family in the village of Ysternia, the island of Tinos.

His father died three months before his birth, and his mother was soon forced to go to Athens and work as a cook to sustain her family.

He rose quickly in the ecclesiastical hierarchy, being appointed second secretary of the Holy Synod, becoming presbyter and archimandrite on 23 June 1940.

Ieronymos was elected as Archbishop of Athens on 14 May 1967 after the forced resignation of his predecessor, Chrysostomos II, following the military coup d'état of 21 April 1967.

Due to his well-known staunchly conservative and anti-communist views, he was widely seen as nominee of the junta's leadership, many of whom had ties to para-religious organizations.