In a land area of 27,120 square kilometers, the diocesan territory covers 12 civil districts of Muzaffarpur, Vaishali, Samastipur, Sitamarhi, Darbhanga, Madhubani, Saharsa, Begusarai, Madhepura, Khagaria, Sheohar and Supaul.
The diocese is located in the northern part of Bihar state having its headquarters in Muzaffarpur, which is also the headquarters of the civil district of Muzaffarpur. The diocese shares a 100 kilometer-long border with Nepal in the north. It borders the Diocese of Purnea in the east, the Diocese of Bettiah in the west. The river Ganges in the south separates it from the Archdiocese of Patna.
The genesis of the diocese can be attributed to the Congregation of the Faith's erection of the prefecture of Tibet-Hindustan in 1703 entrusting it to the Capuchin Fathers of the Italian Province of Picenum in the Marches of Ancona. Father John Francis of Camerion, OFM Cap, was assigned the tutelage of the prefecture as its Prefect.
One of the Capuchin missioners, Father Joseph Mary, happened to spend some time in Bettiah in 1740 en route Tibet and cure the queen of Bettiah of some serious malady. The King of Bettiah, Dhruva Singh, asked the priest to stay in his Kingdom, but the priest expressed his inability to do so unless the Vatican granted permission for the same.
Subsequently, King Dhruva Singh wrote two letters to Pope Benedict XIV requesting that the missioners be allowed to open a station in Bettiah. The pope conceded the request of the Bettiah King and replied him on May 1, 1742 that he was allowing the Capuchins to stay and preach the Gospel in his kingdom. Father Joseph Mary thus founded the Bettiah Mission in 1745.
In 1820, the Vatican elevated the Prefecture of Tibet-Hindustan to a Vicariate with its headquarters at Agra, currently located in the northern Indian state of Uttar Pradesh. In 1827 an independent Patna Vicariate comprising Bettiah, Chuhari, Patna City, Danapore, Bhagalpur, Darjeeling, Sikkim, Nepal and some other adjacent territories was constituted by a papal decree. Father Anastasius Hartmann, OFM Cap, was nominated its Vicar and ordained Bishop of Agra.
Patna Vicariate's first diocesan priest, Father Cajetan Casary of Bettiah, was ordained priest in Rome in 1861.
In 1886, the North Bihar Mission comprising its four stations of Bettiah, Chuhari, Chakhni and Latonah was entrusted to the tireless Capuchins. In May 1892, the Vatican formed the new Prefecture of Bettiah-Nepal to be led by Father Hilaion OFM Cap of Abtei as its Prefect. Twenty years of intensive apostolic activities witnessed the opening of several stations such as Darbhanga, Khorea, Marpa, Muzaffarpur and Samastipur.
The Prefecture was dissolved in 1919 and attached to South Bihar (now Jharkhand state) to form the Diocese of Patna with its first Bishop, Louis Van Hoeck, SJ of Ranchi Mission. Subsequently, Father Augustine Wildermuth, SJ was ordained the third Bishop of Patna in October 1947. In March 1980, the Patna diocese was bifurcated into Patna and Muzaffarpur dioceses. Bishop John Baptist Thakur, SJ became the first prelate of the newly erected Muzaffarpur diocese. In August 1998 the Muzaffarpur diocese was further divided into Muzaffarpur and Bettiah dioceses.
The total population of the diocese is 28,304,000, comprising 14,756,000 males and 13,548,000 females.There is a tribal community known as "Santhal" (Austric stock) and one semi-tribal community called "Mushahars" (Austric stock too) and the non-tribal groups that comprise Hindus, Muslims, Catholic, Sikh, Jain and Buddhists.
Maithili and Hindi are the major languages of the territory. But even Santhali, Bajika, Urdu and English are used.
The per capita income in the diocesan territory is 3,600 rupees (some US$82). The region is extremely flood prone affecting agriculture to a great extent. Still paddy, maize, jute and vegetables are the major agricultural produces. Sugar cane is also produced. Fishery is an important economic activity of the region. Except a few sugar mills industries are virtually absent in the region. Majority of the working adults are migrant laborers working outside the region to eke out a living.
The literacy rate is 37.2 percent.