John Joseph Kennedy was born in Dublin, Ireland on July 15, 1968. On July 13, 1993 he was ordained a Catholic Priest in Dublin, Ireland, in July 1993 . 

Monsignor Kennedy was appointed as a curate in the sprawling parish of Crumlin for three years before he took up an appointment in the inner city parish of St Nicholas of Myra on Francis Street for two years. Former parishioners still continue to visit him in Rome, and he is known as a welcoming host, with interests which vary from sports to cooking .

Fr. Kennedy studied Licentiate in Canon Law and a Specialization in Jurisprudence at the Pontifical Gregorian University from 1998- 2002.  In January 2003 Fr. Kennedy was assigned to the Congregation for the Doctrine of the Faith. In 2006 Fr. Kennedy was named Protocol Officer in the Congregation. Fr. Kennedy was made a  Monsignor on December 17, 2007. He completed his Doctorate in Canon Law and Jurisprudence degree in 2010.  

In 2014 Monsignor Kennedy was appointed Financial Administrator in the Vatican. In this position, Monsignor Kennedy had responsibility for the overall budget for the Congregation for the Doctrine of the Faith. 

In 2015 Monsignor Kennedy was appointed head of the Matrimonial Section, a position he held simultaneously with that of Financial Administrator. 

Pope Francis then appointed Monsignor Kennedy as head of the disciplinary section of the Congregation for the Doctrine of the Faith in 2017.  

Since April 2022, Monsignor is the new secretary of the disciplinary section at the Congregation of the Doctrine of the Faith, which has responsibility for dealing with credible allegations against clergy.

In 2018 he became a Board member of DCYIA.

In September 2024 he was ordained Archbishop  at St. Peters Basilica, Vatican City