He was the second illegitimate son of John Montagu by Martha Ray; he was acknowledged by his father, and brought up at Hinchingbrooke, Huntingdonshire.
[1] On 30 January 1789 he was admitted as a member of Gray's Inn, but continued to reside at Cambridge until 1795, when, having by a technical loophole lost the portion of inheritance intended for him by his father, he came to London to read for the bar.
[2] He was on intimate terms with Samuel Taylor Coleridge and William Wordsworth, whose early enthusiasm for the ideas of the French Revolution he shared.
By Sir James Mackintosh, whose acquaintance he soon afterwards made, and with whom he went the Norfolk circuit, he was converted to political moderation and the study of Francis Bacon.
Montagu never became eminent as a pleader, but he gradually acquired a practice in chancery and bankruptcy; his leisure time he devoted to legal and literary work.
[2] He was a member of the Athenæum Club, and his town house, 25 Bedford Square, was for many years a centre of reunion for London literary society.
[2] Along with Sir James Mackintosh, Thomas Fowell Buxton, William Wilberforce, Richard Martin MP and the Reverend Arthur Broome, Montagu attended a meeting on 16 June 1824 at Old Slaughter's Coffee House in St. Martin's Lane, London that created the Society for the Prevention of Cruelty to Animals (in 1840 by royal assent from Queen Victoria it became the RSPCA).
[2] He published in 1809 a volume of selections entitled The Opinions of different Authors upon the Punishment of Death, London; and in subsequent years a variety of pamphlets on the same topic.
(Sugden was later Lord St. Leonards), followed in 1829; and in 1831 The New Bankrupt Court Act, arranged with a copious Index and Observations upon the Erroneous Principle on which it is Founded, London, 1831.
[2] In 1837 Montagu published, with Scrope Ayrton, The Law and Practice in Bankruptcy as altered by the New Statutes, Orders, and Decisions, London, 2 vols.
Thomas Babington Macaulay criticised Montagu in a celebrated Essay on Bacon, originally published in the Edinburgh Review for July 1837.
Montagu's edition was effectively superseded by James Spedding's work from 1860; he was assisted in it by Francis Wrangham and William Page Wood, who were responsible for the translations of the Latin treatises.
Other works were: Montagu married three times: He had by his first wife a son Basil Caroline, mentioned in William Wordsworth's lines 'To my Sister' and 'Anecdote for Fathers'.
Thomas Carlyle, introduced to her by Irving in 1824, corresponded with her; and during the earlier years of his residence in London was a frequent visitor at 25 Bedford Square.