EXCLUSIVE: For Leslie Carroll, it's easy to understand why seeing Prince Philip on stage was one of the best nights of her life.

It was October 2005 and the royal author was attending a black-tie gala dinner at London’s Guildhall as part of a week-long celebration honoring the bicentenary of the Battle of Trafalgar – known as the legendary British naval victory against a combined French and Spanish fleet under Admiral Lord Nelson. 

The event was a fundraiser for the Seafarers’ Charity, formerly Seafarers UK, of which the Duke of Edinburgh was a royal patron. The nonprofit supports those who’ve worked at sea and have fallen on hard times.

"Philip rose and spoke for 20-25 minutes with tremendous passion and clarity, and without a single note in front of him," Carroll recalled to Fox News. "Everyone in attendance – from the women in gowns to the men in tuxedos to those who wore their naval uniforms and medals – were gobsmacked that the then-84-year-old duke could deliver a lengthy speech with such content and clarity that never for a moment sounded like someone had written it for him, or that he’d memorized it."

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In this June 8, 1948 file photo, the Duke of Edinburgh seated beside Princess Elizabeth, acknowledges the cheers of the crowd as the open Landau passes through Fleet Street on way to the Guildhall in London. (AP)

"It sounded spontaneous and straight from the heart," Carroll shared. "And without a single dicey gaffe – for which he had become legendary."

Philip, the tough-minded husband of Queen Elizabeth II who spent more than seven decades supporting his wife in a role that both defined and constricted his life, died on Friday. He was 99.

The duke was Britain’s longest-serving consort who fulfilled more than 20,000 royal engagements to boost British interests at home and abroad. He headed hundreds of charities, founded programs that helped British schoolchildren participate in challenging outdoor adventures and played a prominent part in raising his four children, including his eldest son, Prince Charles, the heir to the throne.

Carroll has written several books about the British royal family over the years including "Notorious Royal Marriages" and most recently, "American Princess: The Love Story of Meghan Markle and Prince Harry." She described Philip as "the model for the royal husband who supported his wife in everything and subordinated his ego to the royal requirements of a consort."

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"He also gave up the only other thing he loved besides his wife and queen – his career as a naval officer," said Carroll.

Prince Philip completed at least 22,000 royal engagements since his wife’s coronation. (AP Photo/Alastair Grant)

"[But] his passion for the sea never left him, however land-bound he became," she continued. "That night in London’s Guildhall – a building built in 1440 that stands as a reminder that ‘there will always be an England’ – the duke was master and commander again; but this time on behalf of the charitable organization that brings comfort to those former seafarers who find themselves cast upon financial shoals."

Philip saw his sole role as providing support for his wife, who began her reign in 1952 at age 25. Giving up a promising naval career to become consort when Elizabeth became queen, Philip wasn’t content to stay on the sidelines and enjoy a life of ease and wealth.

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Instead, he promoted British industry and science, espoused environmental preservation long before it became fashionable, and traveled widely and frequently in support of his many charities.

Queen Elizabeth and Prince Philip tied the knot in 1947. They remained together until his death on Friday. (Photo by Jeff Spicer/Getty Images)

When Philip turned 90 in 2011, he told the BBC that he was "winding down" his workload, adding he had "done my bit." In 2017, Philip announced he was stepping back from royal duties and stopped scheduling new commitments. 

"I somehow irrationally expected that even at 99, Prince Philip would still be with us," Carroll admitted. "Her Majesty writes a personal letter to every subject who reaches his or her 100th birthday. I’d really been looking forward to imagining what little emendation she would add to her husband’s letter in June. Sadly, we’ll never find out."

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Philip is survived by the queen, 94, their four children, as well as eight grandchildren and 10 great-grandchildren. 

The Associated Press contributed to this report.