He was born the younger son of James Rennie, a farmer near Phantassie, near East Linton, East Lothian, Scotland. He showed a taste for mechanics at a very early age, and was allowed to spend much time in the workshop of Andrew Meikle, a millwright and the inventor of the threshing machine, who lived at Houston Mill on the Phantassie estate. After receiving a normal basic education at the parish school of Prestonkirk Parish Church, he was sent to the burgh school at Dunbar, and in November 1780 he matriculated at the University of Edinburgh, where he remained until 1783. His older brother George remained to assist in the family agricultural business, achieving notability in this arena.

Rennie seems to have employed his vacations in working as a millwright, and so to have established a business on his own account. At this early date the originality of his mind was exhibited by the introduction of cast iron pinions instead of wooden trundles. In 1784 he took a journey south for the purpose of enlarging his knowledge, visiting James Watt at Soho, Staffordshire. Watt offered him an engagement, which he accepted. After a short stay at Soho he left for London in 1784 to take charge of the works at the Albion Flour Mills, Blackfriars, for which Boulton & Watt were building a steam-engine. The machinery was all designed by Rennie, a distinguishing feature being the use of iron instead of wood for the shafting and framing. About 1791 he started in business as a mechanical engineer on his own account in Holland Street, Blackfriars, whence he and his successors long conducted engineering operations of vast importance. (In the same year, the Albion Flour Mills were destroyed by arson.)

The distinguishing characteristics of Rennie’s work were firmness and solidity, and it has stood the test of time. He was most conscientious in the preparation of his reports and estimates, and he never entered upon an undertaking without making himself fully acquainted with the local surroundings. He was devoted to his profession, and, though he was a man of strong frame and capable of great endurance, his incessant labours shortened his life. He was elected Fellow of the Royal Society of Edinburgh on 28 January 1788, Fellow of the Royal Society on 29 March 1798, and in 1815 he served as a Manager of the newly built London Institution.

In 1790 he married Martha Ann Mackintosh (d.1806), daughter of E. Mackintosh, and by her had seven children, two of whom, George and John, became notable engineers. His daughter Anna married the architect Charles Cockerell.

In 1791, he moved to London and set up his own engineering business, having by then begun to expand into civil engineering, particularly the construction of canals. His early projects included the Lancaster Canal (started 1792), the Chelmer and Blackwater Navigation (1793), the Crinan Canal (1794-1801), Rudyard Lake (1797) and the Rochdale Canal, which passes through difficult country between Rochdale and Todmorden (1799). The Kennet and Avon Canal — including the Dundas Aqueduct, Caen Hill Locks and Crofton Pumping Station — occupied him between 1794 and 1810.

In 1802 he revised the plans for the Royal Canal of Ireland from Dublin to the Shannon near Longford. He also served as advisor to Dublin Corporation’s Pipe Water Committee, for which he was presented with the Freedom of the City of Dublin in 1804.

One of John Rennie’s last projects was the construction of the Custom House Docks in Dublin, along with its locks and warehouses, including the CHQ Building where he pioneered the use of cast-iron in the early 19th century. Rennie was first invited to work on the scheme in 1809 by John Foster, the Irish Chancellor of the Exchequer. The first stone of the docks was laid in May 1817; they were formerly opened at the end of August 1821 in front of ‘a most select company of Noblemen, Bishops, Ladies, &c.’ In 1824 the docks were placed on a long-term lease to Harry and John Scovell, and their nephew George. Harry and John were the younger brothers of Sir George Scovell, the intelligence officer famed for cracking Bonaparte’s secret codes during the Napoleonic Wars.

Of all Rennie’s works, that which appeals most strongly to the imagination is perhaps the breakwater at Plymouth Sound, consisting of a wall a mile in length across the Sound, in about 20 metres (66 ft) of water, and containing 3,670,444 tons of rough stone, besides 22,149 cubic yards (16,934 m) of masonry on the surface. It was constructed to provide safe passage for naval vessels entering the river Tamar (Hamoaze) at Devonport. This colossal work was first proposed in a report by Rennie, dated 22 April 1806; an order in council authorising its commencement was issued on 22 June 1811, and the first stone was deposited on 12 August following. The work was completed by his son, Sir John Rennie, and by Joseph Whidbey.

By March 1820, Rennie was seeking 33 tons of structural cast-iron, along with a large quantity of wrought iron, for the purpose of building a ‘Tobacco Warehouse, with the Spirit Stores under it.’ The iron was supplied by the Butterley Iron Company from Derbyshire. However, an obituary of Shropshire-born engineer and iron founder William Hazeldine from 1841 claimed that Hazeldine also supplied ‘the Iron Roofs for the Dublin Custom House and Store Houses.’ In 1821, John James Macgregor noted: ‘The tobacco stores have been finished on the south side at the expense of £70,000. They are 500 feet long by 160 feet wide. The roof is of cast iron, and the building finished in the most permanent manner.’ In 1821, the Rev. George Newenham Wright, an Anglican clergyman, likewise noted:

Rennie was also responsible for designing and building docks at Hull, Liverpool, Greenock, London (London, East India and West India docks), and Leith and improving the harbours and dockyards at Chatham, Devonport, Portsmouth, Holyhead, Ramsgate, Sheerness, Howth and Dunleary. He devoted much time to the preparation of plans for a government dockyard at Northfleet, but they were not carried out. Rennie’s last project was London Bridge, still under construction when he died in 1821 but completed by his son, also John Rennie.

Dunleary harbour of ‘Asylum’ was a very difficult and important project, because it was critical to maintain an effective communication link between Ireland and London, the seat of government. Rennie was responsible for the construction of Howth Harbour on the North side of Dublin bay a decade earlier. This was originally planned as the landing for the Holyhead packets, but it silted up to such an extent that it became unfit for purpose. An Act of Parliament of 1816 (56 Geo.III. Cap 62) authorised the building of Dunleary harbour. Originally it was intended that only one pier (the East Pier) would be built (3,500 feet long), but when John Rennie was appointed directing engineer for the work, he insisted that a single pier would result in sand drifting behind the pier and that a second West Pier (4,950 feet long) would prevent this from occurring. He was correct as the sand has built up behind the west pier. The harbour once built was renamed ‘The Royal Harbour of Kingstown’ in 1821 on the occasion of the visit of George IV. The material for the harbour is Dalkey Hill granite. The granite was provided by Richard Toucher (a long time campaigner for the new harbour) at no cost to the construction team. The foundations of the piers are 300′-0″ wide and 24′-0″ below low water level. Many options were considered for the width of the space between the two pier heads. Rennie wrote to the Harbour Commissioners that the opening should be 430′-0″ wide with the pier heads turned into the harbour to control swells within the harbour. His demands were never met and the harbour opening was built at 1,066′-0″. This was clearly too wide and was subsequently reduced to 760′-0″.

The foundation stone of the new harbour was laid by the Marquess of Downshire on 1 August 1821. The initial plans and surveys for this ambitious undertaking were made by John Rennie. He, however, died within two months of work beginning, and was succeeded by his son, John, who had as his resident engineer a fellow Scot, the seasoned marine builder, David Logan, who had assisted Robert Stevenson at the Bell Rock Lighthouse (1807–1810). The new harbour had to have greater depth to accommodate steam packets. Rock blasted from the sea bed, within the harbour area and further south in what became known as the Quarry Hole at Meetinghouse Point was used to form the outer slopes of the two piers; but the inner faces were built of limestone from the Moelfre quarries of Anglesea. This ‘Anglesea marble’ lends itself to the finest ashlar dressing and the new piers remain a triumph of stone carving. The flights of steps display special skill in the deep diagonal binding of each solid step, providing a typically robust engineer’s response to the wear of seaboots and waves alike. The harbour consists of two independent piers running north westwards out to sea; parallel nearer the shore, they converge at the outer ends to form a harbour mouth 150 feet (46m) wide. At low tide the water in the harbour is fifteen feet deep.

The Holyhead Mail Pier Light is a conical white house which was built by Rennie in 1821. It is probably the second oldest lighthouse in Wales, after Point of Ayr Lighthouse. The lighthouse is of national significance as one of Rennie’s surviving works. Of particular importance, in a Welsh context, is the early date of the lighthouse lantern, which was originally lit by gas. Before the conversion to electricity a gas works was located on the island to power the lighthouse, the piers and even part of Holyhead itself. The works were constructed at a cost of £130,000, an astronomical sum at the time. The tower survives intact and has beautifully curving gallery railings, similar to those at Bardsey Lighthouse. It is no longer in use, although it is used as a navigation reference for sailors.

He died, after a short illness, at his house in Stamford Street, London, on 4 October 1821, and was buried in St. Paul’s Cathedral.

The Bell Rock Lighthouse, near the entrance to the Firths of Forth and Tay, was built during 1807 and 1810. Rennie was, by some, credited with the design and execution, but there seems little doubt that he was only nominally responsible for the great undertaking. Robert Stevenson, surveyor to the Commissioners of Northern Lights, drew the original plans and, at his suggestion, the commissioners called Rennie to assist with obtaining parliamentary approval for the project, giving him the title of chief engineer (for which however he was only paid £400). Stevenson did not accept many of the modifications proposed by Rennie, but the two men remained on friendly terms. Rennie visited the lighthouse twice while it was being built. When Stevenson died in 1850, the Commissioners put on record in their minutes that to him was ‘due the honour of conceiving and executing the Bell Rock lighthouse’. However, Rennie’s son, Sir John Rennie, claimed in a long exchange of letters with Alan Stevenson in 1849 that the advice which Rennie gave Stevenson entitled him to rank the building as one which he “designed and constructed”.