5 December 1885: Onitsha established as a mission with Father Joseph Emile LUTZ C.S.Sp. as the head of the mission.
7 April 1889: Became a Prefecture with Father LUTZ as the first Apostolic Prefect.
28 September 1905: Father Joseph Ignatius SHANAHAN C.S.Sp. was appointed Apostolic Prefect.
20 April 1920: Erected Vicariate of Southern Nigeria by Pope Benedict XV.
6 June 1920: Episcopal ordination of Joseph Ignatius SHANAHAN C.S.Sp.as Apostolic Vicar of the Vicariate of Southern Nigeria. Principal Consecrator: Bishop Denis Kelly of Ross, Ireland.
27 May 1927: Episcopal ordination of Charles HEEREY C.S.Sp. as the coadjutor Bishop of the Vicariate. Principal Consecrator: Bishop Joseph Ignatius SHANAHAN C.S.Sp.
28 April 1950: Vicariate raised by Pope Pius XII to the status of Archdiocese and Metropolitan See of Onitsha Ecclesiastical Province with Bishop Charles HEEREY C.S.Sp. as the first Archbishop.
29 August 1965: Episcopal ordination of Francis ARINZE as coadjutor and first indigenous bishop of Onitsha Archdiocese. Principal Consecrator: Archbishop Charles HEEREY C.S.Sp.
26 June 1967: Appointment of Bishop Francis ARINZE as Archbishop of Onitsha.
11 May 1985: Episcopal ordination of Stephen EZEANYA as the 2nd indigenous Archbishop of Onitsha. Principal Consecrator: Cardinal Francis ARINZE.
6 May 1995: Enthronement of Bishop Albert OBIEFUNA (formerly bishop of Awka) as Archbishop of Onitsha.
9 February 2002: Episcopal ordination of Valerian M. OKEKE as coadjutor Archbishop of Onitsha. Principal Consecrator: Archbishop Osvaldo PADILLA.
1 September 2003: Most Rev. Valerian Okeke became the Archbishop of Onitsha, and the Metropolitan of the Ecclesiastical Province of Onitsha.
28 May, 2007: Granted a Basilica
The Holy Trinity is the Patron of the Archdiocese.
