Sir Hugh Montgomery built a large stone quay to accommodate vessels ferrying between Scotland and Ireland from 1616 onwards.[3] Viscount Montgomery's harbour (1626; improved 1640), superseding what had hitherto been probably only a small jetty, was built and maintained as a result of the Royal Warrant of 1616 which limited travel between the Ards and the Rhins of Galloway to this port, and that at Portpatrick also owned by Montgomery. It was described by Harris in 1744 as 'a curving quay about 400 feet (120m) long and 22 feet (6.7m) wide built of uncemented stones'. It ran from the shore at the north end of the Parade in a broad arc, bent against the open sea, towards the southern end of the present north pier. Much patched and decrepit, the quay was virtually rebuilt, though along the original line, between 1775 and 1785 by the landlord, Daniel Delacherois, probably with the help of John Smeaton, the distinguished civil Engineer who had apparently made earlier more elaborate plans for extending the harbour, and who had just rebuilt Portpatrick harbour. The old quay remained until after the completion of the new harbour, and then, despite its continued favour by local fishermen, was removed for local wall building about 1833 (it appears in the 1832 drawing but not on the first O.S. map of 1834).

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. For many years he was engaged in extensive drainage operations in the Lincolnshire and Norfolk fens (1802–1810), and in the improvement of the River Witham. The Eau Brink Cut, a new channel for the River Ouse, was completed just before his death.

Rennie was a man of unbounded resource and originality. During the improvement of Ramsgate harbour he made use of the diving-bell, which he greatly improved. He is generally credited with the invention of a form of steam-dredging machine with a chain of buckets, but in this he seems to have been anticipated by Sir Samuel Bentham. He was certainly the first to use it on an extensive scale, which he did during the construction of the Humber Dock, Hull (1803–09), when he devised a steam dredger to overcome the difficulties of that particular work, and apparently without any knowledge of Bentham's invention. Another expedient was the use of hollow walls, which was suggested by the necessity of providing an extensive bearing surface for the foundations of a wall in loose ground. Walls built upon this plan were largely used by Rennie.

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.

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".

Over the next few years Rennie also attained a deserved reputation as a builder of bridges, combining stone with new cast-iron techniques to create previously unheard-of low, wide, elliptical arches, at Leeds Bridge, and in London at Waterloo Bridge (1811–1817), with its nine equal arches and perfectly flat roadway (thought to be influenced by Thomas Harrison's design of Skerton Bridge over the River Lune in Lancaster). His later efforts in this line also show that he was a skilful Architect, endowed with a keen sense of beauty of design. Waterloo Bridge was considered his masterpiece and was the most prestigious bridge project in England, described as 'perhaps the finest large masonry bridge ever built in this or any other country'. The Italian Sculptor Canova called it ‘the noblest bridge in the world’ and said that ‘it is worth going to England solely to see Rennie’s bridge.’ London Bridge, built from his design by his sons, though not constructed until after his death, replaced the medieval bridge which was proving a serious impediment to the flow of the river and was eventually moved to Arizona. Southwark Bridge (1815–1819) was built as three cast-iron spans over the river. He also designed the Old Vauxhall Bridge.

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". It is worth noting that the Bay formed between Kingstown Harbour and The Forty Foot is still known as Scotsman's Bay after Rennie.

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.

Donaghadee is probably best known for its lighthouse and harbour. For centuries, it has been a haven for ships, and the harbour has been there from at least the 17th century.