On June 24, 1972, Lahham was ordained a Catholic priest in Jerusalem. He later became Fidei donum in Dubai and vicar and priest in Jordan.
In 1992, Lahham received his PhD in pastoral theology and catechesis in the Pontifical Lateran University. In 1994, he was appointed rector of the Latin Seminary in Beit Jala.
On September 8, 2005, Lahham was appointed Bishop of Tunis by Pope Benedict XVI and was ordained on October 2 of the same year in the parish church of Beit Jala by Latin Patriarch of Jerusalem, Michel Sabbah.
On May 22, 2010, Pope Benedict XVI raised the Diocese of Tunis to an Archdiocese and gave to Lahham the title of Archbishop of Tunis.
Less than two years later, on January 19, 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 February 6, 2017, Pope Francis accepted the resignation of Lahham as Auxiliary Bishop of Jordan.[1]