Marshman was the first child of Joshua Marshman and Hannah Marshman and was born in August 1794 at Bristol, England where his father was at that time a schoolmaster, before later emigrating to India as a missionary. He died at Radcliffe Square, North Kensington, on the 8 July 1877.
In May 1800, his parents opened two boarding schools at Serampore; these became the most popular in the area and Marshman received his education from his parents. He was part of the growing mission family, eating at the communal table and joining with other children in Mission life; as one would expect he became a fluent Bengali speaker.
In 1875 the Friend of India amalgamated with another paper The Englishman, becoming The Statesman which remains one of India's leading English-language dailies.
Marshman also started a new paper mill at the Mission in order to manufacture a special new type of paper that had been devised by the missionaries to resist the virulent ravages of the local white ants. This became known as "Serampore Paper" and was used throughout the province.
In 1820 a steam engine was imported from Messrs. Thwaites and Rothwell, of Bolton, Lancashire, for the paper mill and was the first ever seen in India. Marshman's father Joshua was mesmerised by it and watched closely as the engineer prepared it for use.
Marshman joined the staff of Serampore College, which had been jointly founded by his father, in 1821.
In 1837 the last of the "Serampore Trio", his father Joshua Marshman died. Following his death John Mack and Marshman struggled to carry on the work of the College, spending all their earnings and Marshman's income from his private concerns, including those from the paper mill. After he published The Friend of India, he stipulated that the proceeds should go to the College. It was reckoned that in all he contributed more than £30,000.
As the struggle to maintain the College was getting more onerous each year to try and fund privately, Mack and Marshman decided to turn the College over to the Baptist Missionary Society. The Society was unwilling to take over the burden fully, but did offer to support a theological professor on the college staff.
Marshman later rather unwillingly accepted the position of Official Bengali Translator to the Government, and thereafter was abused almost daily in the native newspapers as "the hireling of the Government". His salary of £1,000 per annum was passed to the College.
In 1855 Marshman planned to leave India for good. Mack and he proposed once again to pass control of the College to the Baptist Missionary Society; this time the proposal was accepted. He resigned his post as Official Bengali Translator to the Government and returned to England to Kensington Palace Gardens.
Marshman was a devoted student of Indian history and he wrote what was for many years the only history of Bengal. He was also long engaged on the writing of the history of India; his reading was very wide and he was a distinguished Oriental scholar. He studied Chinese (like his father) and knew all the great Sanskrit poems. He also gave much attention to Persian.
In England, however, he was not recognised and was refused a seat on the Indian Council, and through his services to education, he was tardily recognised by the granting of the Star of India in 1868. In order to earn a living he had to seek the position as chairman of the Committee of Audit of the East Indian Railway.
On his death it is said that he had known as much about Indian affairs as if he had been the personal assistant to four successive Viceroys.
He also published 'Marshman's Guide to the Civil Law of the Presidency of Fort William' which was possibly one of the most profitable law books ever published.