The area under the Diocese of Kottar had known the Christian Faith even several Centuries before the arrival of the Portuguese on the West Coast. Monuments recently excavated in a place called Chinna-Muttom amply testify to this fact. The presence of St. Thomas Christians in places like Thiruvithancode and Kottar, presently parishes in the Diocese, has been attested by several missionaries under the Padroado.
However with the coming of the Portugese missionaries a small section of the local people, known as Paravas, became Christians. About 7 years after their conversion, in December 1544, over ten thousand fishermen, known as Mukkuvas, living on the sea coast from Poovar to Pallam were baptized by one of the greatest missionaries of modern times, St. Francis Xavier.With the renewed efforts of the Sacred Congregation for Propagation of Faith (founded in 1622) Christian missionaries began to penetrate into the interior parts of the area. Initially attention was given to the conversion of the Nairs. The 18th century saw rapid growth of Christianity in the interior parts of the area, following a large-scale conversion of people belonging to the Nadar Community.
The great Martyr of the Diocese, Devasahayam was born in 1712, converted to Christian faith in 1745 by the gentle efforts of one captured captain of the Dutch Navy, called E.B. De Lannoy in 1752 and killed for the Christian faith at Aralvaimozhy. His mortal remains have been interred in the church of St. Francis Xavier, which in 1930 became the Cathedral of the new Diocese of Kottar.
The Church of Kottar came under the efficient supervision of the Carmelites. The Carmelite Bishop of Quilon, Most Rev Aloysius Maria Benziger organized and strengthened the existing Church of Kottar. The present diocese of Kottar was created on 26.05.1930 with the Most Rev Lawrence Pereira as its first Bishop.
Most Rev Thomas Roch Agniswamy succeeded him in 1938 and eventually detached the Diocese from the Metropolitan See of Veropoly and attached it to the Archdiocese of Madurai on 11.10.1963.
Most Rev Marianus Arokiasamy succeeded him in 1971. He was committed to the implementation of the reforms of Second Vatican Council, especially in the fields of liturgy, catechetic, Bible and pastoral activity. He was promoted as Archbishop of Madurai in 1987.
Most Rev Leon A. Tharmaraj, the fourth Bishop, who assumed office in 1989 and who hails from this Diocese faithfully carried forward the polices of his predecessor and went on further in making the Diocese a Church of the laity, concentrating on the formation of BCCs and Pastoral Councils and encouraging lay leadership in the mission of the Diocese.
Following the death of Bishop Leon on January 16, 2007, Bishop Peter Remigius of Kumbakonam was transferred and installed as bishop of his native diocese Kottar on August 24, 2007.