Prior to being elected as Catholicos-Patriarch, Mar Addai II served as Bishop of Baghdad. He was later elevated to the rank of Metropolitan of Iraq in September 1968, after the schism separating the Ancient Church of the East from the Assyrian Church of the East. Together with Mar Aprem Mooken and Mar Poulose Poulose, Mar Addai consecrated Mar Thoma Darmo as the Patriarch of the Ancient Church of the East in October 1968.
Mar Thoma Darmo died the following year in September 1969, falling one month short of the first anniversary of his consecration as Catholicos-Patriarch. Mar Addai II was then selected to serve as Acting Patriarch.
In December 1969, as Acting Patriarch, Mar Addai II elevated Mar Narsai Toma to Metropolitan of Kirkuk and Mar Toma Eramia as Metropolitan of Mosul and Northern Iraq. He was officially elected to the position of Catholicos-Patriarch in February 1970, several months after the death of Mar Thoma Darmo. Two years later, on February 20, 1972, he was consecrated as Patriarch of the Ancient Church of the East by Mar Narsai and Mar Toma. The ordination took place at St. Zaia Cathedral in Iraq.
The Ancient Church of the East has seen many changes during Mar Addai's tenure, namely the consecration of a number of new Bishops and Metropolitans. Since his elevation to the rank of Patriarch, the church has consecrated the following prelates: Mar Daniel Yakob (Auxiliary Bishop of California), Mar Yacoub Daniel (Metropolitan of Australia and New Zealand), Mar Emmanuel Elia (Bishop of Baghdad - resigned), Timothaus Mar Shallita (Metropolitan of Europe), Mar Zaia Khoshaba (Bishop of Baghdad), Mar Aprem Daweed (Bishop of Duhok - resigned), Mar Mari Emmanuel (Bishop of Australia and New Zealand), and Mar Gewargis Younan (Bishop of Chicago).
Between November and December 1995, Mar Aprem Mooken and Mar Poulose Poulose left the Ancient Church of the East for external reasons, and reunited with the Assyrian Church of the East. Mar Addai II has also shown interest in reuniting the sister churches. A meeting between both Holy Synods is expected in the near future, as the Ancient Church of the East has made gestures towards reconciliation. The most prominent of these is undoubtedly the declaration made in June 2010 in which Mar Addai II announced that the church would officially recognize Christmas on December 25 starting that year, in accordance with the Gregorian Calendar. This was a historic move, as calendar issues were one of the primary reasons leading to the schism.
Despite this move, the Holy Synod of the Ancient Church of the East has maintained that they will continue celebrating Easter in accordance with the Julian Calendar.
On Sunday, September 24, 2006, St. Mary's Cathedral, the home of Mar Addai II, was bombed. The cathedral, located in the Riyadh district of Baghdad, experienced dual bombings: A small IED explosion preceded a car detonation setting off a large number of explosives. The bombings occurred within minutes of each other. The bombing was carefully timed to occur just as parishioners exited the church following Sunday morning services. Two civilians were killed in the attacks, in addition to the twenty that sustained injuries.
The Patriarch was also attacked on another occasion in his own courtyard. Though his life was spared, he was robbed.
Mar Addai II's tenure as Patriarch comes at arguably the darkest time for Assyrian Christians since the Assyrian Genocide also known as Seyfo. Mar Addai has stressed the importance of maintaining the Assyrian Christian presence in Iraq. Despite attacks against his life and the threat facing all Christians in the country, he has refused to relocate and continues to reside in Iraq.
On May 18, 2007 Mar Addaii II called on the Prime Minister of Iraq and the Iraqi Parliament to put an end to the tragic situation Christians in Iraq were forced to endure as a result of constant threats and assaults against them.