Hieronymus was born in the village of Ysternia, on the island of Tinos in 1905, the son of Hieronymus Kotsonis who died three months before his son was born. The future Archbishop's mother was forced to work in Athens as a cook leaving her son on the island with relatives. After two years, little Hieronymus was able to join his mother and received his education in the Greek capital. He eventually studied at the Theological School of the University of Athens and, thanks to a scholarship, furthered his studies in Germany and England.
On January 4, 1939, Hieronymus was ordained a deacon. On June 24, 1940, he became a priest and served in the ministry of hospital visitation during the German ocuupation. After World War II, he became priest to the Royal Palace and at the same time was appointed professor at the Aristotle University of Thessaloniki.
On May 14, 1967, Hieronymus was elected Archbishop of Athens and all Greece however, due to political pressures of that time, he was soon forced to resign. He was replaced on January 13, 1974 by Archbishop Seraphim.