The diocese of Ajmer comprises of twelve districts namely Ajmer, Jodhpur, Kota, Barmer, Bundi, Jalore, Jhalwar, Pali, Sirohi, Tonk, Jaisalmer and Baran in the State of Rajasthan.
In July 1890 the Holy See detached a very large portion of the Archdiocese of Agra to make a new unit, the Rajasthan Mission, entrusting it to the Capuchin Fathers of the Paris Province who were to work under the supervision of the Archbishop.
In March 1892, the new mission became the Prefecture Apostolic of Rajputana, and in 1913, the then Prefect Rt. Rev. Fortunatus Henry Caumont, OFM. Cap. was appointed the first Bishop of Ajmer. On March 11, 1935 parts of the Dioceses which were in the present state of Madhya Pradesh, were detached to become part of the newly erected Prefecture Apostolic of Indore.
The Diocese was handed over to the Diocesan clergy on May 9, 1949. By the decree of the Holy See dated June 21, 1951. Bandikui with its sub-station Alwar was transferred for geographical and administrative reasons from Agra Archdiocese to Ajmer Diocese. However the Agra Archdiocese retained the jurisdiction over the districts of Bharatpur and Dholpur.
By another decree of the Holy See dated May 13, 1955 the name of the diocese was changed to Ajmer-Jaipur. By decree of the Sacred Congregation for Evangelisation of People (Prot.no.1332/81 dated March 11th 1981) the civil district of Mandasaur was transferred from the jurisdiction of the diocese of Ajmer-Jaipur to that of the Diocese of Indore.
On December 29, 1984 the Holy Father Pope John Paul II, with a decree, divided the Diocese of Ajmer-Jaipur, forming a new diocese named the Catholic Diocese of Udaipur consisting of the districts of Bhilwara, Chittaurgarh, Udaipur, Dungarpur, Banswara and Thandla tehsil. The Holy Father through the same decree appointed Rt. Rev. Joseph Pathalil as it's first Bishop.
Through a mutual agreement between the diocesan bishop and the Superior General of the Little Flower Congregation, Cochin dated April 25, 1986 the districts of Sri Ganganagar, Hanumangarh, Churu, Bikaner, Jaisalmer, Jhunjhunu, Karoli and Sikar were entrusted to the Little Flower Congregation to carry out the work of Evangelisation, pastoral ministry and development on a permanent basis.
On July 20, 2005, the Holy Father Benedict XVI, with a decree bifurcated the Diocese of Ajmer-Jaipur forming a new Diocese – The Catholic Diocese of Jaipur consisting of the District of Jaipur, Alwar, Bikaner, Churu, Dausa, Sikar and Sri Ganganagar and appointed Most Rev. Oswald Lewis as the bishop of the New Diocese, while the name of the mother Diocese was changed to the Diocese of Ajmer.
- Most Rev. BishopCaumont :1913-1930
- Most Rev. Bishop Pius :1931-1938,
- Most Rev. Bishop Guido :1939-1946
- Most Rev. Bishop Leo D'Mello : 1949-1979,
- Most Rev. Ignatius Menezes : 1979-2012.
- Most Rev. Pius Thomas D'Souza : 19th Jan 2013.