Sir John Rennie (1794-1874), the younger son of John Rennie, Civil Engineer, was born in London. He constructed the second London Bridge to his late father’s design and was knighted in 1831 at its opening. In 1825 Rennie and his older brother George became joint Engineers in chief of the Liverpool and Manchester Railway. He worked for the Surrey, Sussex, Hants, Wilts and Somerset Railway Company. He was also appointed Engineer for the Direct Independent London and Manchester Railway, which was never completed. In 1852 he sketched out a system of railways for Sweden, and in 1855 a series of unexecuted railways and harbours for Portugal

Rennie followed his father to become an engineer to the Admiralty. He carried out various works at Sheerness, Woolwich, Plymouth, Portsmouth, and Ramsgate. Rennie was primarily a hydraulics engineer, and much of his career was spent in adding to or altering commercial harbours and docks. In Britain these included important docks at Whitehaven and Cardiff; abroad, he built the Ponte Delgada breakwater for orange-trade boats in the Azores. He completed the drainage works in the Lincolnshire fens commenced by his father and, in conjunction with Telford, constructed the Nene outfall near Wisbech (1826–31). He also restored the harbour of Boston (Lincolnshire) in 1827–8. Rennie also contributed to J. and G. Rennie shipbuilding yard in Greenwich.

Rennie was elected a fellow of the Royal Society in 1823 and of the Zoological Society in 1825. He joined the Institution of Civil Engineers only in 1844 and was its president in 1845–8. He retired about 1863 and died at Bengeo, near Hertford.