King Charles III's ancestors were not just of royal birth, they also ruled the country as prime ministers.
King Charles III's fourth great-grandfather was William Cavendish Bentinck, the 3rd Duke of Portland. Portland served as the Prime Minister twice. The first time was in 1783 when he was Prime Minister of Great Britain and the second time was after a gap of 26 years when he became Prime Minister of the U.K. from 1807 to 1809.
However, he was a true member of the British nobility who held every title including that of duke, marquess, earl, viscount, and baron. His political affiliation was the Whig party.
During Portland's tenure, the American revolutionary war ended. He was also politically active during the French Revolution. During his political career, he also helped suppress the Irish Rebellion of 1798.
The Irish Rebellion of 1798 was inspired by the American and French Revolutions. The Irish sought to obtain parliamentary reform and elimination of British rule in Ireland. British troops aggressively put down the rebellion and many Irish rebels were transported to penal colonies in Australia.
In 1809, while he was still prime minister, Portland started to suffer from ill health and resigned. He died shortly after in 1809 at the age of 71.
Portland's great-granddaughter was Cecilia Bowes Lyon - mother of Queen Elizabeth the Queen Mother (King Charles III's maternal grandmother).
The title of "Duke of Portland" has now become extinct. The last Duke of Portland, Bill Bentinck died in 1990. Like King Charles III, Bill Bentinck was descended from the 3rd Duke of Portland. This would make the both of them distant cousins.