Smith went to India as a young man to be principal of Doveton College, Calcutta, a school for Eurasian pupils. After five years he became editor of the Calcutta Review and the Friend of India. Though not a missionary himself, throughout his time in India he had a deep interest in mission and missionaries, and he and his wife, Janet, were ever-welcoming hosts for traveling missionaries. His interest in Indian society and Anglo-Indian relations led to his later being appointed as a Companion of the Order of the Indian Empire by Queen Victoria.
Returning to Scotland in 1875, he became secretary of the foreign mission committee of the Free Church of Scotland in 1870, a post he held for more than 30 years. He became a senior statesman in the Scottish churches and was influential in encouraging a wider interest in and support for mission.
He also wrote widely on historical and geographic topics, and his books of missionary biography (e.g., Carey, Duff, Martyn and Van der Kemp) were popular in the late nineteenth century.