Macnaghten, Sir William Hay (1793–1841), diplomat, was born in Calcutta on 24 August 1793, the second child of the six sons and ten daughters of Sir Francis Workman Macnaghten, first baronet, barrister, of Dundarave, Co. Antrim, and Letitia, eldest daughter of Sir William Dunkin of Clogher. He was educated at Charterhouse School, London, and in 1809 travelled to Madras as a cadet in the East India Company. From 1811 to 1812 he served in the 4th cavalry at the nizam's court in Hyderabad, gradually acquiring proficiency in a number of Eastern languages, including Hindustani, Persian, Telegu, Kanarese, and Marathi. In 1914 he entered the Bengal civil service and held a succession of legal posts in Bengal, Malda, and Shahabad. His flourishing career enabled him to marry, on 23 August 1823, Frances McClintock (d. 1878). Macnaghten proved himself to be legally as well as linguistically adept in Eastern culture; his extensive experience of Indian law was reflected in his acclaimed treatises Principles and precedents of Moohummudan law (1825, 3rd ed. 1864) and Principles and precedents of Hindu law (1829, repr. 1865), both of which were accepted as standard authorities by the courts of British India. He also published Reports of cases in the court of Nizamut Adawlut (1827) and Reports of cases in the court of Sudder Dewanny Adawlut (1827), as well as an edition of Arabian nights (4 vols, 1839–42).
Macnaghten's distinguished legal and literary accomplishments have been overshadowed by the notoriety of his subsequent political career. After a tour of the upper Indian provinces with Lord William Bentinck in the early 1830s, he was appointed chief secretary of the secret and political department in 1833. Four years later he accompanied Lord Auckland, then governor-general of India, on a tour of the north-west provinces, becoming one of the latter's closest advisors. Ambitious and self-confident, Macnaghten advocated the British invasion of Afghanistan in 1838 to depose the reigning amir, Dost Mohammad Khan, and install a puppet government in Kabul under the nominal rule of Shah Shujah. This interventionist policy achieved early success – though it was sustained only by British military presence and enormous subsidies paid by the Indian exchequer to the Afghan chiefs – and Macnaghten was appointed envoy and minister to the Afghan court. His career was then at its zenith: he was created a baronet (18 January 1840), appointed a provisional member of the council of India (September 1840), and nominated governor of Bombay (September 1841).
Shujah's tenuous hold on power in Afghanistan was exposed in November 1841, when the Afghan chiefs mounted a revolt at Kabul which the British army, disorganised and disgruntled, was unable to combat. Macnaghten was compelled to negotiate a political solution which amounted to a complete British withdrawal from Afghanistan and the restoration of Dost Mohammad. As the treaty began to go awry, Macnaghten held a separate parlay with Akbar Khan (the son of Dost Mohammad), but this, it emerged, was a stratagem of Khan's to discredit Macnaghten and demonstrate supposed British duplicity. At a meeting on Seeah Sung Plain on 23 December 1841, Akbar assassinated Macnaghten. His body was mutilated by the people of Kabul, but his remains were later recovered and buried in Calcutta, where there is a monument to his memory.
An archive of his correspondence is held in the British Library, London; a portrait (1841) by James Atkinson is in the National Portrait Gallery, London.