Seaward was born the son of a builder in Lambeth, London, in January 1786, and initially worked with his father as a surveyor and architect.
In this part of the country John became friendly with Arthur Woolf, Richard Trevithick, and other mechanical engineers of the period.
In 1829 they assisted in the formation of the Diamond Steam Packet Company, and built the engines for the boats which ran between Gravesend and London.
The saving obtained in the consumption of fuel by the double-slide valve, both for the steam and exhaust, plus other improvements, caused the government to entrust the Seawards with the building of twenty-four steamboats and some smaller vessels.
They were also early advocates of the use of auxiliary steam power for the voyage to India, and experimented with it on the East Indiaman Vernon in 1839 and 1840 with great success[3] They also designed large swing bridges, dredging machines, cranes, and other dock apparatus, plus machinery for lead, saw, and sugar mills.
Among the improvements and inventions for which John Seaward was personally responsible were tubular boilers, which were used by the Royal Navy, disconnecting cranks for paddle-wheel engines, the telescopic funnel, self-acting nozzles for feed and for regulating the saturation of the water in marine boilers, double passages in cylinders both for steam and education, cheese-couplings used to connect and disconnect screw propellers to and from engines, and other minor improvements.