Mathias was head of the Salesian missionary expedition to
Assam. Leaving Italy in 1921, he reached
Shillong in January 1922. On 12 December 1922 he was nominated Prefect Apostolic of Assam, Bhutan and Manipur. He was also the first Salesian Provincial of India, an office that he held until 1934. On 10 November 1934, he was nominated as the first bishop of the new diocese of Shillong. It was Mathias who initiated the idea of a Salesian presence in Bombay (
Mumbai). The idea was put to him in a letter from an influential Catholic of Bombay, Mr. F.A.C. Rebello, dt. 29 March 1923. The decision was taken in 1927, and the Salesians took over the Immaculate Conception School for Goan boys at Tardeo in 1928. This school was under the
Padroado bishop of Daman. Since the Propaganda Archdiocese of Bombay had objected to the entry of the Salesians, this was how Msgr. Mathias circumvented the opposition. ==Archbishop of Madras and Mylapore==