He was an agriculturist and a friend of the agricultural writer Arthur Young, and was an enthusiastic canal builder who invested in many commercial ventures for the improvement of his estates.

Charles Greville assessed him as "immensely rich and his munificence was equal to his wealth" and wrote that "in his time Petworth was like a great inn."

[2] In 1774, he added O'Brien to his name on inheriting extensive estates in Ireland from his uncle Percy Wyndham-O'Brien, 1st Earl of Thomond.

[3] Wyndham was a patron of painters such as Turner and Constable, and of the sculptor John Flaxman who contributed a heroic group of Michael overthrowing Satan for the North Gallery.

Like his father, the earl also collected French furniture, as when on a visit to Paris in July 1802 during the Peace of Amiens, he bought a pair of five-light candelabra supported by bronze female caryatids,[4] supplied by Martin-Eloy Lignereux.

[5] The earl was an enthusiast for canal building, which would allow for agricultural improvement on his Petworth estates by bringing in chalk from Houghton for liming, and coal to replace scarce supplies of firewood, releasing more land for food production.

Failing, during the time of Canal Mania, to find any reliable contractor able to undertake the construction, most of the work was done by the earl's own estate workers.

[7] In 1796, the earl purchased 36 per cent of the shares in the Arun Navigation Company, saving it from bankruptcy when it was burdened with the £16,000 cost of building the Coldwaltham cut and Hardham tunnel.

[8] Having abandoned plans for a canal from Petworth to Shalford and keen for the nation to have an inland waterway linking London and Portsmouth, safe from natural hazards to coastal shipping and naval attack by the French, the earl turned his attention to linking the River Arun to the River Wey in Surrey.

[11] War with France and population growth made famine an ever-present danger in the early nineteenth century and there was an urgent need to maximise food production using any land that could be cultivated.

The earl established a pedigree herd of Sussex cattle from the local breed which was commended by Young who wrote that they "must be unquestionably ranked among the best of the kingdom".

More unusually Young describes opium production at Petworth, with juices from the incised poppy heads being scraped into earthenware bowls and dried in the sun.

The Earl of Egremont, with a spirit of liberality which pervades all his actions, gives to farmers, in the neighbourhood of Petworth, the opportunity of breeding from his valuable stud; his lordship also affords the eastern part of the county the same opportunity, by giving the use of one of his best bred horses to Mr. Brown, the venerable training groom at Lewes; his lordship also gives annual premiums to the breeders of the best colts, shewn at Egdean fair, near Petworth.In 1800, Wyndham bought land at Houghton, Sussex, where he developed chalk pits, which in 1808 Arthur Young reported as producing 40,000 tonnes (40,000,000 kg) annually.

[20] Near Northchapel a government factory was set up to produce high-quality charcoal for making gunpowder from alder wood in coal-heated iron cylinders.

At Spofforth in North Yorkshire, the geologist William Smith was employed to search for coal and iron ore deposits.

[23] When the party split in 1792 over the French Revolution he sided with the more conservative faction which supported Prime Minister William Pitt the Younger in his condemnation of the "wicked and seditious" writings of radicals such as Thomas Paine.

When the Sussex Poor Law Commissioner William Hawley visited Petworth House in October 1835 he was politely received but was informed that the earl considered the Act "one of the worst measures that could have been devised".

The national press reported the matter and Sockett, together with other witnesses from Petworth, gave evidence to a House of Commons select committee in March 1837.

[25] The county Yeomanry was revived in 1794 "in case of invasion or internal commotion", reflecting aristocratic nervousness following the French revolution.

[30] The Market House or Town Hall was built of stone and adorned at the northern end with a bust of William III.

[32] Thirty-two cells in two storeys were built over brick arch arcades to prevent tunnelling out, and the institution opened in 1788 near the present police station and courthouse.

Lamb often spent time at Petworth House as a child and continued to visit Egremont until the end of the earl's life.

Petworth and various estates in Yorkshire and Ireland passed to Colonel George Wyndham, the eldest natural son of the third Earl, who in 1859 was created Baron Leconfield.