On his father's death in 1807, John inherited his share of the company and developed the business, becoming sole owner of the works in 1815. In 1817 he married Maria Rankin but their marriage was short lived, Maria dying just nine months later. He subsequently married Lady Charlotte Elizabeth Bertie, daughter of Albermarle, ninth Earl of Lindsey.
Guest was elected member of parliament for Honiton, Devon in 1825, holding the seat until defeated by Sir G. Warrander in 1831. His public works included a school at Dolwais, designed by Sir Charles Barry. He was a Fellow of the Royal Society and a Member of the Institution of Civil Engineers.
He died in 1852, leaving five sons and five daughters, the Dolwais Iron Company having become the largest producer of iron in the world.