In a land area of 21,051 square kilometers, the diocesan territory covers the four districts of Mysore, Mandya, Coorg and Chamarajanagara. It is surrounded by the Archdiocese of Bangalore, Diocese of Ooty, Calicut, Mangalore and Chikmagalur.
In the territory of Mysore diocese, the population is 5,940,262 at end of 2009. (The population of the whole country is 1.2 billion).
The Mission of Mysore was separated from the Vicariate Apostolic of Pondicherry on March 16, 1845, to become first pro-Vicariate, and then in 1850 a Vicariate Apostolic entrusted to Msgr. Charbonnaux who was, till then, Co-adjutor to the Bishop of Pondicherry.
In 1886, the Vicariate of Mysore was elevated to a Diocese, and its Vicar Apostolic, Msgr. Coadou became its First Bishop, residing, however, in Bangalore. Until 1940, the Diocese of Mysore included the entire districts of Mysore, Coorg and Kollegal, but on Feb. 13, 1940, by the Apostolic Letter "Felicius Increscente" of Pius XII, Bangalore was separated from Mysore and the Bishop of former Mysore Diocese Bishop M.B. Despatures became the Bishop of Bangalore, comprising the districts of Bangalore, Kolar, Tumkur and Chitradurga.
Originally Mysore belonged to the Archdiocese of Goa, but what early mission work was done there is a matter of obscurity. In the Canarese or western portions a mission seems to have been established about the middle of the seventeenth century; in the eastern or Telugu portion another mission was brought into existence about the year 1703 by two French Jesuits who came from Vellore and founded churches at Bangalore, Devanhalli, Chikka, Ballapoora, and elsewhere. But their work was stopped and partly destroyed by the fanaticism of the sultan, Tipu (1782-99). The district came under the Foreign Mission Society of Paris in 1776, which at that date began work at Pondicherry. The celebrated Abbé Dubois (b. 1765, d. 1848), himself a member of the Foreign Missions, spent most of his life among the Canarese Christians of Ganjam, Palhally and Satthully (see DUBOIS). Mysore was included in the Vicariate of the Coromandel Coast (Pondicherry), erected in 1836, but was separated in 1845, and erected into a distinct Vicariate-Apostolic in 1850, at the same time as the district of Coimbatore. On the establishment of the hierarchy in 1886 it was made into a diocese suffragan to Pondicherry with the same boundaries as now.
The Diocese of Mysore (as distinct from Bangalore) was reconstituted in 1941 with the addition of the Nilgiris district and part of the Coimbatore district.
However 15 years later, on July 3, 1955, by the Apostolic Constitution "Nuntiatur in Psalmis" of Pope Pius XII, these districts of Nilgiris and Coimbatore were again separated from Mysore and erected into Diocese of Ootacamund (Ooty) in Tamil Nadu.
On Nov. 16, 1963, by the Apostolic Constitution "Indicae Regionis Conditio" of Pope Paul VI, three most districts, Chikmagalur, Hassan, Shimoga, were separated from the Mysore Diocese to form the Diocese of Chikmagalur.