[3] On February 12, 1733, El Khazen was ordained bishop by Maronite Patriarch of Antioch Jacob Awad.

In 1743 at the death of his uncle Patriarch Joseph Dergham Khazen, after the initial refusal of Simon Awad, Tobias in competition with Elias Mohasseb tried to be elected patriarch, but the case came to the Pope who chose Simon Awad in place of the two pretenders.

[2] In 1755 he was appointed Eparch of Tripoli and at the death of Simon Awad, Tobias succeeded at being elected patriarch on February 28, 1756.

[7][8] The new patriarch placed his residence in Kisrawan, particularly in the monastery of Mar Rouhana.

The dispute over the Lebanese Maronite Order could be solved only under his successor, Joseph Estephan, when in 1770 it was divided into two branches, giving rise to OMM.