In 1839 he was appointed professor of moral and political economy in the Hindoo college at Calcutta; became one of the editors of the Englishman newspaper there, and edited the government Gazette while Lord Auckland was governor-general (1837–41).
In 1847 Johnson commenced a series of works called The Gardener's Monthly Volume, the first portion of which, on the potato, was written by himself.
On the death of his father-in-law, Newington Hughes, banker, Maidstone, Johnson succeeded to his property, when the Fairfax manuscripts came into his possession.
These valuable documents, which had been rescued from a shoemaker at Maidstone, were in 1848–9 published as the Fairfax Correspondence in four large volumes, the first two of which were edited by Johnson, the last two by Robert Bell (1800–1867).
Johnson died at his residence, Waldronhurst, Croydon, on 29 October 1886, and was buried in the grounds of St. Peter's Church on 4 November.