In 1992, Lahham received his PhD in pastoral theology and catechesis in the Pontifical Lateran University.

On 8 September 2005 Lahham was appointed Bishop of Tunis by Pope Benedict XVI and was ordained on 2 October that year in the parish church of Beit Jala by Latin Patriarch of Jerusalem, Michel Sabbah.

Less than two years later, on 19 January 2012, Lahham was recalled to the Middle East where he became Auxiliary Bishop and vicar of the Latin Patriarchate of Jerusalem for Jordan.

He received the title of Titular bishop of Madaba and maintained, as an individual, the dignity of archbishop.

On 6 February 2017 Pope Francis accepted the resignation of Lahham as Auxiliary Bishop of Jordan.