Buckingham Palace released the news alongside an official portrait of the couple.
Lady Gabriella is the daughter of Prince and Princess Michael of Kent, the Queen’s first cousin.
Mr Kingston, who goes by the name of Tom with his friends and family, is a friend of Prince William and previously had a relationship with Pippa Middleton, the sister of Kate, the Duchess of Cambridge.
The couple dated in 2011 and remained friends after their split, being spotted on numerous social occasions, even after Pippa married James Matthews in 2017.
The Bristol University educated Mr Kingston formerly helped secure the release of hostages in Iraq with the Iraqi Institute of Peace (IIP), before embarking on a career in finance at Schroders private bank.
There he was an equity analyst before becoming managing director of an emerging markets investment company. He is currently a director at a financial insights company.
Lady Gabriella, known as Ella to family and friends, is a senior director at Branding Latin America, a brand-positioning platform that specialises in the region. She is also a writer and contributes to The London Magazine.
Lady Gabriella and Mr Kingston is likely to marry her fiance in spring next year, the palace indicated.
Buckingham Palace said in a short statement: “Prince and Princess Michael of Kent are delighted to announce the engagement of their daughter Lady Gabriella Windsor to Mr Thomas Kingston.
“The engagement took place in August; Mr Kingston proposed on the Isle of Sark.”
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