Ieronymos succeeded the late Archbishop Christodoulos, who died of cancer at his home near Athens in late January at the age of 69.
Ieronymos, 70, was one of the main contenders for the position in the previous 1998 election, but lost out to Christodoulos.
The new head of the Greece's Orthodox Church is expected to be less prone to confrontation than Christodoulos, who had often accused liberals of trying to dilute Greece's Christian identity.
Christodoulos was known as an outspoken conservative and defender of the church's role in state affairs, as well as a fierce critic of globalization and Turkey's bid to join the European Union.
He held a historic meeting with Pope John Paul II during his visit to Greece in 2001. He also met with the Catholic leader's successor Pope Benedict XVI at the Vatican in 2006 as part of efforts to end the Great Schism between the churches, dating back to 1054.
The Holy Synod is the Greek church's decision-making body.
