The Diocese of Diphu was erected on 5th December 1983, detaching the District of Karbi Anglong from the Archdiocese of Shillong-Guwahati, and the District of North Cachar Hills from the Diocese of Silchar. Most Rev. Mathai Kochuparampil, SDB was appointed the first Bishop of the new Diocese. He was installed at Diphu on 4th March 1984. He passed away on 4th March 1992. Rev. Fr. Albano D’Mello,SDB was elected Diocesan Administrator on 11th March 1992. On 24th June 1994, Most Rev. John Thomas Kattrukudiyil was appointed the second Bishop of the Diocese. He was consecrated at the Cathedral of the Risen Lord, Diphu on 8th September 1994. He was transferred to the newly formed Diocese of Itanagar in December 2005. However, he  continued to serve the Diocese of Diphu as its Apostolic Administrator until a new Bishop was appointed. On 14th February 2007, Most Rev. John Moolachira was appointed the third Bishop of the Diocese. He was consecrated on 15th April 2007 at the Cathedral of the Risen Lord, Diphu. He was transferred to the Archdiocese of Guwahati to be its Auxiliary Archbishop; and he left the Diocese of Diphu on 14th June 2011. On 16th  June  2011, Rev. Fr. John Timung was elected by the college of Consultors as the Diocesan Administrator of the diocese of Diphu. On 26th July 2013, Most Rev. Paul Mattekatt was appointed the fourth Bishop of the Diocese. He was consecrated at the Cathedral of the Risen Lord, Diphu on 6th October 2013. 

​The oldest missionary presence in the Diocese is that of the Sisters of Our Lady of the Missions (RNDM) at Haflong. The first group of sisters settled in Haflong in 1911. They started a small boarding school, which later on was opened to the children of the British engineers and tea garden managers. But due to malaria and other fatal diseases prevalent in the region, the infant school had to be closed. In March 1918, St. Agnes School was re-opened by the sisters. Soon a priest began to reside at Haflong and served as chaplain to the convent. According to a government letter, the school was meant for the Europeans and the Eurasians in the province and not for any general missionary purpose. Such restrictions remained till the Independence of India.

​In 1927, with the election of Msgr. Lepailleur, CSC as the first Bishop of Chittagong, Cachar was made part of the new diocese. In 1930, Bishop Lepailleur acquired a fairly large plot of land adjacent to the convent on lease. In 1943, he obtained another lease on a piece of land about 10 minutes walk from the convent, which later became the headquarters of the Prefecture of Haflong and the present residence of priests.

​The Haflong mission was initially run by the Holy Cross Fathers (CSC) attached to the Canadian Province of the Congregation. On 7th January 1952, Haflong was raised to the status of a Prefecture Apostolic and Msgr. Gomes Breens CSC, was appointed the first Prefect Apostolic. His area of jurisdiction consisted of Cachar Hills, Mizo Hills and Tripura. The Haflong Prefecture Apostolic was raised to the status of a Diocese on 26th June 1968 with the Episcopal See at Silchar, and Most Rev. Denzil D’Souza was appointed its first Bishop.

In July 1974, Rev. Fr. Peter Bianchi, SDB was appointed the parish priest of Haflong. With the erection of the Diocese of Diphu, the parish of Haflong, which comprised the whole district of North Cachar Hills, became part of it. The Catholic community of the district of North Cachar Hills is made up of Karbi, Zema Naga, Khasi, Adivasi, Mizo, Hmar, Dimasa and other tribal groups.

​The Salvatorian Fathers (SDS) had adopted Khasi Hills as their main field of missionary activity. They reached the boundary of Karbi Anglong but did not really enter it. Mr. Langtuk Hanse from the village of Marjong, just a few Kilometers from the present parish centre of Umswai, was the first to hear about the Gospel. On 25th January 1914, he led a group of six people from Marjong to Umtyrkhang in Khasi Hills and received baptism at the hands of Fr. Chrysostomus Lefef Mayr, SDS. These six in turn became apostles, and on 4th May 1916, 31 others from the same village received the Catholic faith. On the 15th of the same month, another 22 received baptism. In 1920, some of those who had received baptism came to settle in Umpanai (Amkachi); thus a community was started there too. Later on a Catholic community was formed in Mynser. The work in this region was not restricted to the Karbis only. The Tiwas too responded to the faith. In 1950, some people of Bor Marjong and Umswai embraced the Catholic faith.

​Mr. Joseph Milik and his four children were the first to receive the Catholic faith in Block I under Jrikyndeng. They went to the Raliang Mission and were baptized on 6th March 1942. They were the people of Umkhyrmi. From here the faith spread to the Karbis of other villages.

​Mr. John Kathar with a group of 27 others were the first to receive the Catholic faith in the Rongkhang area. Msgr. Orestes Marengo, SDB received this first group into the Church.

​The first baptisms in Dokmoka region among the Karbis were at Dentaghat in 1969. In 1950, some Catholics came from Sojong to Diphu to settle there, and thus a community was started there too. Later on, the Karbis at Japralangso, Rihalangso, Balipathar and other villages were contacted. The first baptisms in Chokihola region were in the year 1973.

​Although the first baptisms in Karbi Anglong were in 1914, the first parish  was opened only in the year 1967, at Sojong. On 11th February, the parish of Sojong was inaugurated with Fr. John Mariae Nedumpettil as the first parish priest. In 1971, the parish of Diphu was opened. In 1972, the parish of Dokmoka was opened. The parish of Chokihola was opened in 1974, and Umswai in 1977. Japrajan was the last parish opened (1982) before the erection of the Diocese of Diphu.

​Today, the Diocese has 30 Parishes, 14 Mission centres, 58 Convents. The missionary team of the Diocese comprises the diocesan priests, religious priests, brothers and religious woman belonging to various congregations, numerous full time catechists, village catechists and youth leaders. The Diocese is constantly endeavoring to move forward in the spirit of the first missionaries who planted the Gospel in this part of the world.

Christianity in the new diocese can be traced back to the Augustinian Fathers from Dhaka. Rangamati Dhubri district had a very vibrant Christian community in the 16th century. In the last century the Salvatorian priests and later the Salesians while traveling through the Brahmaputra had stayed at Dhubri.

However, the first to take up residence at Dhubri were the Salesian Fathers Archimede Piannazzi and L. Rocca, who were appointed to work among Garos. In 1932 they took up residence at Dhubri and used it as a base for their apostolate in the Garo Hills as Catholics were denied permission to work in that area. In 1936 Barpeta Road Mission was established. The whole area of the present diocese and more was covered by this parish. The present progress can be attributed to Bishops Orestes Marengo, Joseph Mittathany, Robert Kerketta and to Fr Remo Morra, sdb. Under the leadership of Archbishop Thomas Menamparampil, the area had a further growth which led to the creation of the Bongaigaon diocese. Fr Joseph Zubizzaretta, sdb’s name will be always remembered among the most outstanding missionaries of the area.

The Diocese of Bongaigaon is carved out of the Archdiocese of Guwahati consisting of Bongaigaon, Baksa,Barpeta, Chirang, Dhubri Kokrajhar and Nalbari Districts of lower Assam that covers the surface area of 13,630 Sq Km. with an over all population of 5.2 million of which 56,855(1.2%) are catholics. There are 27 parishes, 28 Diocesan Priests, 36 Religious Priests, 07 Brothers and 164 Women religious. Catholic population is mainly tribals with the Bodos forming the single largest group. Santhals, Adivasis, Garos and Rabhas are the other major groups. General population comprises of Koch and Koch Rajbanshies, the Assamese and Bengalis. Dhubri and Barpeta districts have Muslim majority.