After leaving school, John (junior) worked with his father surveying routes for projected railway lines in England.
Mather, Dixon and Company closed in 1843 and Grantham began a practice as a Naval Architect and Consulting Engineer.
He was involved in the design of several large iron sailing and steam ships, including Sarah Sands, Pacific, Antelope and Empress Eugenie.
In 1830, Grantham won a prize from the London and North Western Railway for a design for pulling up passenger carriages from Lime Street to Edge Hill Station, Liverpool, by stationary engines.
He took out several patents for screw propellers and invented a system of sheathing iron-built ships with wood and copper, to prevent fouling in tropical climates.