Mitra joined Calcutta Public Library as deputy librarian in 1836. The library was established the same year in the residence of an Englishman named Strong in the Esplanade. It was later shifted to
Fort William College and when the Metcalfe Hall was constructed to pay respect to the memory of
Charles Metcalfe, the library was moved to the hall in 1844. Mitra rapidly rose up the ladder as librarian, secretary and finally curator, a position he held till his retirement. He was associated with a variety of social welfare activities of his day. He was a member of the Calcutta University Senate, the society for prevention of cruelty to animals, and the Bethune Society. He was secretary of the British India Society (later Association). He was also a
Justice of the Peace. He had a strong interest in the development of agriculture in the country. His criticism of the permanent settlement,
The Zemindar and Ryots, created a sensation. While a member of the Agricultural Society, he started an organisation for the translation of books on agriculture from English to Bengali. In 1881, when
Madame Blavatsky and
Col Olcott visited India, he became involved with the
Theosophical Society. ==Journalism and literary work==