On May 10, 1964, His Holiness Pope Paul VI raised Tezpur to an Episcopal See, and nominated Rt. Rev. Orestes Marengo SDB DD, Bishop of Dibrugarh, as the Administrator of the newly erected Diocese of Tezpur. He was installed at Tezpur and took charge of the Diocese on August 23, 1964. By his pastoral care, Bishop Marengo was able to consolidate the many new Christian communities and to bring about the over all development of the Diocese. He resigned in 1969. Rt. Rev. Joseph Mittathany was nominated the second Bishop of the Diocese on June 26, 1969, and he was ordained Bishop and installed on September 27, 1969.

The Bhutan formed part of the Diocese till 1976. It was separated and attached to the Diocese of Darjeeling by the Decree “Quo Facilius” (Jan 20, 1975).

John Paul II erected the Diocese of Imphal, Manipur on April, 2, 1980 and transferred Rt. Rev. Joseph Mittathany from Tezpur to Imphal as its first Bishop. The See being vacant, Rev. Fr. Mathew Kottaram was elected the Vicar Capitular. Rt. Rev. Robert Kerketta SDB DD, who had been Bishop of Dibrugarh from 1970, was transferred to Tezpur, and took charge of the Diocese on January 31,1981.

The Diocese of Guwahati was erected by the Apostolic Decree “Opitulante Quidem Domino” (30, March 1992). 15 Centres of the then Diocese of Tezpur in the Civil Districts of Kokhrajhar, Dhubri, Barpeta, Nalbari,  Kamrup, Morigaon and part of the District of  Nagaon were taken to form part of the new Diocese. The Diocese of Guwahati was inaugurated on August 16, 1992. Further Guwahati diocese was again bifurcated and the diocese of Bongaigaon was created on May 10, 2000. Reorganization of the Ecclesiastical Province of Shillong-Guwahati took place on August 1, 1994 and it provided for the creation of 3 Provinces in the region- Guwahati, Shillong and Imphal. Tezpur became a suffragan of Guwahati along with Dibrugarh, Diphu and Bongaigaon

On December 7, 2005 the Holy See announced yet another bifurcation of Tezpur and the creation of the Diocese of Itanagar, Arunachal Pradesh, with Rt. Rev. John Thomas Kattrukudiyil (of Diphu Diocese) as its first Bishop. The Erection of the Diocese and Installation of Bp. John Thomas took place on March 12, 2006.

The Diocese of Tezpur today comprises the Civil Districts of Darrang, Udalguri, Sonitpur, Biswanath, Lakhimpur and Dhemaji on the North bank of the Brahmaputra; Rupahihat, Kaliabor and Samuguri Subdivisions of Nagaon District on the Southern Bank of Brahmaputra. Bp. Robert Kerketta after shepherding the diocese of Tezpur as its Bishop for a long glorious period of 27 years retired on December 3, 2007 and is succeeded by Rt. Rev. Michael Akasius Toppo as its new Bishop.