1842 in the United Kingdom
- 6–13 January – First Anglo-Afghan War – Massacre of Elphinstone's army (Battle of Gandamak) by Afghan forces on the road from Kabul to Jalalabad, Afghanistan, under Akbar Khan, son of Dost Mohammed Khan.
- February – J. H. Newman, the controversial Anglican cleric, withdraws to Littlemore, outside Oxford, and establishes a semi-monastic community, "the house of the Blessed Virgin Mary at Littlemore".
- 31 March – Middleton Junction and Oldham Branch Railway line opened up to Werneth.
- 13 April – first Anglo-Afghan War: British victory at the Battle of Jellalabad.
- April–September – General Strike ("Plug Plot Riots").
- 25 April – Black Country Nailers' Riots.
- 11 May – Income Tax Act establishes the first peacetime income tax in Britain; 7 pence on the pound, for incomes over 150 pounds.
- 4 June – in South Africa, hunter Dick King rides into the British military base in Grahamstown to warn that Boers have besieged Durban. He had set out eleven days earlier. The British Army dispatches a relief force.
- 13 June
- 19 June – army suppresses Rebecca Rioters in Carmarthen protesting against turnpike tolls.
- August–October – first Anglo-Afghan War: British victory at the Battle of Kabul.
- 7–27 August – riots in and around Lancashire (spreading to Yorkshire by around 12 August), protesting against the Corn Laws and in favour of Chartists.
- 9 August – the United Kingdom and United States sign the Webster-Ashburton Treaty agreeing the border between the United States and Canada.
- 10 August – Mines Act prohibits women and children under ten from working in mines.
- 29 August – Britain and Qing dynasty China sign the Treaty of Nanking, an unequal treaty ending the First Opium War. Hong Kong is ceded to Britain.
- Pentonville Prison, London, built.
- Beecham's Pills (a laxative) first marketed in St Helens, Lancashire by Thomas Beecham, forming the basis of the Beecham pharmaceutical company.
- John Kilner & Co., glass manufacturers, established in Castleford, Yorkshire, to produce the Kilner jar.
- New pattern musket with percussion cap produced to replace the flintlock in the British Army.
- Paleontologist Richard Owen coins the term "Dinosaur".
- Antarctic explorer James Clark Ross charts the eastern side of James Ross Island.
- J. M. W. Turner paints Peace – Burial at Sea and Snowstorm - steam boat off a harbour's mouth.
- 14 May – The Illustrated London News first published.
- Edwin Chadwick's critical Report on an inquiry into the Sanitary Condition of the Labouring Population of Great Britain published by the Poor Law Commission.
- Thomas Babington Macaulay's poems Lays of Ancient Rome.
- Alfred Tennyson's collection Poems.
- 13 May – Arthur Sullivan, composer (died 1900)
- 26 July – Alfred Marshall, economist (died 1924)
- 23 August – Osborne Reynolds, engineer and physicist (died 1912)
- 20 September – James Dewar, chemist and physicist (died 1923)
- 12 November – John Strutt, 3rd Baron Rayleigh, physicist, Nobel Prize laureate (died 1919)
- 2 December – Charles W. Alcock, footballer and football official (died 1907)
- 13 March – Henry Shrapnel, soldier and inventor (born 1761)
- 5 June – Thomas Henry Lister, novelist (born 1800)
- 12 June – Thomas Arnold, historian and schoolmaster (born 1795)
- 7 October – Jonathan Backhouse, banker and Quaker minister (born 1779)
- 31 October – Solomon Hirschell, chief rabbi of the United Kingdom (born 1762)
- Willetts, Arthur (1995). The Blackcountry Nailer's Riots of 1842. Dudley Libraries. ISBN 0-900911-36-0.
- Palmer, Alan; Veronica (1992). The Chronology of British History. London: Century Ltd. pp. 264–266. ISBN 0-7126-5616-2.
- Body, Geoffrey (1985). Western Handbook — a digest of GWR and WR data. Weston-super-Mare: British Rail (Western). ISBN 0-905466-70-5.
- Penguin Pocket On This Day. Penguin Reference Library. 2006. ISBN 0-14-102715-0.
- "Icons, a portrait of England 1840–1860". Archived from the original on 17 August 2007. Retrieved 13 September 2007.
- "'Best for Me, Best For You' — a History of Beecham's Pills 1842–1998". The Pharmaceutical Journal. 269: 921–924.