In 1762, he was elected metropolitan bishop of Vize, in 1783 of İzmit, and in 1791 of Derkoi.
[1] On 3 March 1794, he was elected Ecumenical Patriarch, succeeding Neophytus VII of Constantinople.
During his patriarchy, Gerasimus III regulated many ecclesiastic issues.
One of which was setting an age requirement in the ordainment of clerics in 1795, prohibiting the ordainment of deacon under 25 years old and presbyter under 30 years old.
On 19 April 1797, he resigned and retired to Tarabiye, where he died a while later.