He subsequently entered his father's manufactory in Holland Street, Blackfriars Road, where he acquired a practical knowledge of his profession, and in 1813 he was placed under Mr. Hollingsworth, resident engineer of Waterloo Bridge, the foundations of which he personally superintended.

In 1815 he assisted his father in the erection of Southwark Bridge, and in 1819 he went abroad for the purpose of studying the great engineering works on the continent.

Rennie also was responsible for designing Cannon Workshops, which arose following the strike by the coopers employed by the West India Dock Company.

Many years of his life were spent in making additions and alterations to various harbours on different parts of the coast, both in England and in Ireland.

He completed the drainage works in the Lincolnshire fens commenced by his father, and, in conjunction with Telford, constructed the Nene outfall near Wisbech (1826–1831).

Rennie was elected a member of the Institution of Civil Engineers on 25 June 1844, and he became president on 21 January 1845, retaining the office for three years.