From 1717, until he left for Ireland in 1732, Mordecai was rector of
St Katherine Coleman in the city of London. Also commonly spelt St Catherine Coleman, the Church was in poor structural condition while Mordecai was there. Rebuilding began in 1739; it was eventually demolished in 1925/6. Mordecai was, from 1718 to 1724, headmaster of
The King Edward VI School, Morpeth, Northumberland, where he is credited with restoring the school's dwindling prestige after a staff feud prior to his appointment, and increasing the attendance from 26 to 100: he was also perpetual curate of
Jarrow, co. Durham 1722–24. He received his Doctor of Divinity at
Lambeth Palace in London, by
William Wake,
Archbishop of Canterbury; his patent bears the date 22 March 1731. Thereafter, he was appointed chaplain to
Lionel Sackville, 1st Duke of Dorset. According to
The Dublin Gazette, the Duke of Dorset arrived in Dublin on 11 September 1731 and was sworn in as Lord Lieutenant of Ireland, under
George II. On 1 April 1732 in St Anne's Church, Dublin, Cary was consecrated
Bishop of Clonfert and Kilmacduagh, County Galway, which position he held to 1838. He was, in fact, consecrated on 26 March "by the Archbishop of Tuam, assisted by the Bishops of Killala and Elphin; and was enthroned at Clonfert on June 5th, and at Kilmacduagh on June 16th".
Clonfert was the abbey and cathedral founded by
Brendan the Navigator in , and its school became one of the most famous and largest in western Ireland. In 1579,
Elizabeth I wrote to the Bishop of Clonfert regarding a consideration that a university be founded there; Dublin was chosen instead. On 27 September 1735
The Dublin Gazette read: "On Wednesday last arrived here the Duke and Duchess of Dorset with the Rt Hon. Walter Cary, His Grace's chief secretary." On this visit by the Duke, "1735-6 Mordecai Cary DD Bishop of Clonfert, was translated to the Sees by patent dated December 20th and was enthroned by proxy at Achonry on March 19th and at Killala next day". See
Bishop of Killala and Achonry, County Mayo. St Patrick's Cathedral, Killala is also significant in Irish church history in that
St Patrick himself, in the 5th century, declared
Killala, previously known as "the wood (forest) of Foclut" to be the origin of his call to evangelise the Irish. Killala derives from the Irish "Cill Aille" meaning Church of Ailill. Mordecai remained at Killala until his death in 1751. The inscription on his tomb in the floor of St Patrick's Cathedral, Killala reads: :To the memory of the :Rt. Revd. Father in God :Mordecai Cary D.D. :Late Ld Bishop of Killala and Achon :who departed this life :Octr ye 2d A. Dom 1751 :in the 65 year :of his age. :Catherine Cary his beloved wife :caused this stone to be laid. :Near him lye four of his grandchildren. Mordecai's appointment at Killala became a family affair. His son, Henry Cary (1717–1769), became Archdeacon of Killala from 1742, aged 24 or 25. The Dean of Killala at this time was Theophilus Brocas (c.1710-1770) whose son, John, succeeded him as Dean, and whose daughter, Henrietta, married the Archdeacon's son, William.
Henry Francis Cary, translator of Dante's "Divine Comedy", was the son of William Cary and Henrietta Brocas, and the great-grandson of Mordecai Cary, D.D., Bishop of Killala and Achonry. In his will, Cary mentioned only one surviving son, Henry Cary, to whom went all his estate, with the exception of that reserved for his widow's use, namely the house and furniture in Henry St, Dublin. He also left the sum of 20
English pounds, to be paid out of his English Concerns, to his brother, William. ==Portrait of Bishop Mordecai Cary==