The son of John Rennie and brother of Sir John Rennie. George and John the younger took over the family firm forming J and G Rennie which completed their father's schemes at first. In 1830, they worked on the Liverpool & Manchester Railway alongside George Stephenson. They manufactured some locomotives and engines for the Royal Navy; they produced the engines for Archimedes, the world's first propeller-driven steamship. A huge variety of mechanical products, including the first biscuit making machinery, corn and chocolate mills for Deptford victualling yard, and the machinery at the Royal William Victualling Yard, Plymouth.