The third son of William Emerson, a merchant of Belfast, he was born there in 1804. He was educated at the Belfast Academy and Trinity College, Dublin, of which he afterwards became LL.D. He took up the cause of Greek independence, and travelled in Greece, publishing a Picture of Greece (1826), Letters from the Aegean (1829), and a History of Modern Greece (1830); and he was called to the English bar at Lincoln's Inn in 1831. In this year he married the daughter and co-heiress (with her cousin, Robert James Tennent, M.P. for Belfast, 1848-52) of William Tennent, a wealthy merchant at Belfast, who died of cholera in 1832, and he adopted by royal licence the name of his wife in addition to his own.
He entered parliament in 1832 as member for Belfast. In 1841 he became Secretary to the Board of Control, and in 1843 he was presented with a service of plate by the calico printers of Great Britain as an acknowledgment of his getting a bill passed in Parliament for the copyright of calico designs.
In 1845 he was knighted and appointed colonial secretary of Ceylon, where he remained till 1850. While he was there, an economic depression in the United Kingdom severely affected the local coffee and cinnamon industry. Planters and merchants clamoured for a reduction of export duties. Tennent therefore recommended to Earl Grey, Secretary of State for Colonies in London that taxation should be radically shifted from indirect taxation to direct taxation, which proposal was accepted. It was decided to abolish the export duties on coffee and reduce the export duty on cinnamon leaving a deficit of £40,000 Sterling which was to be met by direct taxes on the people. This was one of the causes of the Matale Rebellion of 1848.
The result of his residence in Ceylon appeared in Christianity in Ceylon (1850) and Ceylon, Physical, Historical and Topographical (2 vols., 1859). The latter was illustrated by his protege, fellow Ulsterman Andrew Nicholl. The Oxford English Dictionary attributes to it the first use in English of 'Rogue Elephant', a translation of the Sinhala term hora aliya.
On his return, he became member for Lisburn, and under Lord Derby was secretary to the Poor Law Board in 1852. From 1852 till 1867 he was permanent secretary to the Board of Trade, and on his retirement he received a baronetcy.
In his early years his political views had a Radical tinge, and, although he subsequently joined the Tories, his Conservatism was of a mild type. He withdrew from the Whigs along with Lord Stanley and Sir James Graham, and afterwards adhered to Sir Robert Peel. However, he broke with Peel over the Corn Laws and followed the Derbyites. He died in London on March 6, 1869. His family consisted of two daughters and a son, Sir William Emerson Tennent, who was an official in the Board of Trade, and at whose death the baronetcy became extinct.
Besides the books above mentioned, Emerson Tennent wrote Belgium in 1840 (1841), and Wine: its Duties and Taxation (1855), The Wild Elephant and The Method Of Capturing It in Ceylon (1867) Sketches Of The Natural History Of Ceylon (1868) and was a contributor to magazines and a frequent correspondent of Notes and Queries.