He was the second son of George Tennant (1765–1832), of 62, Russell Square, London, also of Rhydings and of Cadoxton Lodge, Glamorganshire, attorney (in practice at 2, Gray's Inn Square, in partnership with Thomas Green)[1] and landowner, builder of the Neath and Tennant Canal in Glamorganshire, by his wife Margaret Elizabeth, daughter of Thomas Beetson.
Tennant married aged 51;[1] his wife, Gertrude Barbara Rich Collier (1819–1918), was a notable society hostess.
They had a son, Charles Coombe Tennant (1852–1928)- whose wife, Winifred Coombe Tennant (née Pearce-Serocold),[3] was a suffragist and Liberal politician- and five daughters, three surviving to adulthood: Alice (1848–1930), who remained unmarried; Dorothy (1855–1926), who married the explorer Henry Morton Stanley; and Eveleen (1856–1937), who married the spiritualist and classical scholar Frederic William Henry Myers (1843–1901).
Between 1856 and 1869 he wrote and published numerous works, covering such disparate subjects as decimal coinage, Utilitarianism and railways.
[1] He spent his final years living at 2, Richmond Terrace, London, having retired from his legal practice in 1866; on his death his only son, Charles, inherited Cadoxton.