He met his second wife, Lucinia Pinelli, in Rome, while serving on a design committee of the Italian Ministry of Marine.

Working for his father, at first on the Great Grimsby Royal docks, then in company with his elder brother Lewis Rendel on the eastern breakwater and new Admiralty pier at Holyhead, he was well prepared for an apprenticeship to his father's great friend, Sir William Armstrong, at his Elswick works.

He lived with Armstrong at his house in Jesmond for three years before completing his engineering education at his father's London office.

His father died in 1856 and the brothers George, Stuart and Hamilton all joined Armstrong's company, while Alexander took over the family business.

[1] In 1859 Sir William Armstrong formed the Elswick Ordnance Company in order to supply guns for the British Army.

George Rendel was one of seven partners in the new company, and was in joint charge of the ordnance departments, together with Captain Andrew Noble.

He designed a series of 1,350 ton unarmoured 16 knot cruisers for the Chinese (Chaoyong and Yangwei) and Chilean navies.

[3] Rendel and Alfred Yarrow pioneered the use of forced-draught fans in boiler rooms, significantly increasing the power of marine steam engines at minimal cost in weight or volume.

[1] He was persuaded to rejoin Armstrongs in 1888, in order to manage a new armaments factory, built as a subsidiary, at Pozzuoli, near Naples in Italy.

[2] In 1863 he was elected a member of the Institution of Civil Engineers and the following year his paper "Gun carriages and mechanical appliances for working of heavy ordnance"[4] was awarded the Watt medal.