At an early age he went to London as a surveyor under Thomas Telford, by whom he was employed on the surveys for the proposed suspension bridge across the Mersey at Runcorn.

[2] In August 1824 he was employed by the Earl of Morley to make a bridge across the Catwater, an estuary of the Plym within the harbour of Plymouth at Laira.

With the exception of John Rennie's 1819 Southwark Bridge over the Thames, it was the largest iron structure then existing, and in 1836 Rendel received a Telford Medal from the Institution of Civil Engineers for a paper describing its construction.

Between 1832 and 1834 similar floating bridges were erected at Torpoint and Saltash across the Tamar, which greatly facilitated the intercourse between Devon and Cornwall.

During this period Rendel was also engaged in reporting on harbours and rivers in the southwest of England, and thus acquired that mastery of hydraulic engineering on which his fame chiefly rests.

In 1836–37 he designed, as a terminus to the Great Western Railway, the Millbay Docks, Plymouth, afterwards executed by Isambard Kingdom Brunel.

In 1843–44 he constructed canals in Devon, and was engaged in the Colchester and Arundel navigation; and in 1844 he designed harbour improvements for Newhaven and Littlehampton in Sussex.

About 1838 Rendel dissolved his partnership with Beardmore at Plymouth, and settled in London, but still was chiefly employed on work for his native county.

The contest was long protracted, and the incessant labour served to shorten Rendel's life; his published evidence forms a valuable record of engineering practice of the period.

As constructor of the Grimsby docks he was one of the first to apply W. G. Armstrong's system of hydraulic machinery for working the lock gates, sluices, cranes, etc.

Other children include: A nephew, James Murray Dobson, became resident engineer of the Buenos Aires harbour works in the 1880s and 1890s.