Rennie, John (1761–1821), civil engineer, was born 7 June 1761 at Phantassie, near East Linton, Haddingtonshire (East Lothian), Scotland, fourth son of James Rennie, farmer, and Jean Rennie (née Rennie), who is believed to have been a cousin. His elder brother was George Rennie (1749–1828), the noted agriculturist and author. He was educated locally, showed a strong mechanical bias while still young, and was influenced by Andrew Meikle, a mechanical engineer who worked as a millwright on his father's estate. In 1775 he went to Dunbar High School before entering Edinburgh University (November 1780), where he studied until 1783. After a tour of engineering works in England, he initially worked with James Watt in the firm of Boulton & Watt. In 1791 he set up his own engineering firm, basing himself in Blackfriars, London. During the course of his career he was responsible for numerous important works in Scotland and England, including the Aberdeen Canal; the East India Docks and Waterloo Bridge, London; Prince's Dock, Liverpool; Plymouth breakwater; and Holyhead harbour.
In 1802 he began a series of works in Ireland. He was appointed as consultant on the Dublin Bay and harbour improvement scheme and used the map of Lt (later Capt.) William Bligh's 1800 survey of Dublin Bay to draw up a series of plans for improvements in the harbour. After the death (1802) of Richard Evans (qv), consulting engineer to the Royal Canal project, he took over. Finding things in a state of confusion, he resurveyed the line, amended the plans for the canal, and oversaw the new work. In 1803 he carried out a series of improvements on the harbour at Portrush, and in 1807 began the new harbour at Howth; these works were largely completed by 1819. In 1811 he was appointed to design new docks in Dublin port; this began his association with the Custom House Dock scheme, for which he designed new docks, warehouses, and also a gas works and gas lighting system. In 1813 he designed a new entrance arch to the Custom House docks, and in 1818 was in conflict with the wide street commissioners, as their intention to widen Commons St. affected his plans for the dock area. He also designed a series of bonded warehouses or ‘stacks’ which were built to a revolutionary plan, their cast-iron roofs being their most notable feature. These last were built in 1820–21, in time for the official opening of the George's Dock scheme in 1821, during the visit of George IV.
Rennie was also responsible for other projects around Ireland including improvements to Donaghadee, Portpatrick, and Belfast harbours. In 1817 he commenced work on Dunleary (Kingstown; Dún Laoghaire) harbour; the east and west piers were largely completed by 1821, also in time for George IV's visit. He died 4 October 1821 at Stamford St., Blackfriars, London. The Gentleman's Magazine referred to his death as ‘a national calamity’. He was buried in St Paul's cathedral.
He married (November 1790) Martha Ann Mackintosh; they had nine children. Two of his sons, George Rennie (1791–1866) and John Rennie (1794–1874) were also civil engineers. A large collection of documents relating to Rennie's Irish engineering projects is in the OPW papers in the NAI. He is mentioned in the inscription on the George IV obelisk in Dún Laoghaire harbour. His entrance arch to the Custom House Docks was dismantled brick by brick and moved to George's Dock in 1995.
His son Sir John Rennie (1794–1874), civil engineer, was born 30 August 1794 at 27 Stamford St., Blackfriars Road, London. He was educated at the schools of Dr Greenlaw at Isleworth and Dr Burney at Greenwich, and then went to work with his father at his engineering firm in Holland St. He showed considerable engineering skill while still a relatively young man, and in 1813 supervised the laying of the foundations of Waterloo Bridge. In 1815 he collaborated with his father on the Southwark Bridge project and then made a tour of continental engineering works. After the death of his father (1821) he went into partnership with his elder brother, George, and took responsibility for the civil engineering side of the business. He succeeded his father as engineer to the admiralty (1821–62), and in October 1821 applied to the commissioners of the Irish board of works to be appointed directing engineer of the Kingstown harbour project, which was still unfinished. The board did not appoint him until 1826, by which time it was obvious that the ends of the east and west piers were too far apart. A distance of over 1,000 ft (305 m) separated them, and in strong easterly gales sections of the harbour could not be used. Rennie pointed out that his father's original design provided for a much smaller entrance between the ends of the two piers. He was supported in this by Sir Francis Beaufort (qv), hydrographer to the admiralty, but work did not restart until 1842. In 1822 he was appointed directing engineer of the Howth harbour project and it is likely that he supervised the building of the last of the bonded warehouse ‘stacks’ in the Custom House docks. He was elected FRS in 1823.
He is, perhaps, best remembered for building London Bridge, which he completed to his father's design in 1831. In recognition of his work, he was knighted on 10 August 1831, the first engineer to be so honoured since Sir Hugh Myddelton. Elected a member of the Institute of Civil Engineers (June 1844), he later served as the institute's president (1845–8). He also published several books including Historical, practical and theoretical account of Plymouth breakwater (1848) and Theory, formation and construction of British and foreign harbours (1851). He retired in 1862 and died at Bengeo, near Hertford, on 3 September 1874. His autobiography was published posthumously (1875). There is a collection of his papers in the OPW collection in the NAI which includes draft plans of the Howth and Kingstown projects.