John Fremantle was born on 22 January 1927, the son of John Walgrave Halford Fremantle, 4th Baron Cottesloe (1900–1994) and Lady Elizabeth Harris, daughter of James Edward Harris, 5th Earl of Malmesbury and Dorothy Gough-Calthorpe.
Both his parents were descendants of American Loyalist members of the Dutch Schuyler and Van Cortlandt families of British North America.
He followed the family tradition set by his ancestor Admiral Thomas Fremantle and joined the Royal Navy in 1945.
A justice of the peace for Buckinghamshire from 1984, he resided at Swanbourne, where he was commonly referred to as 'The Commander'.
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