The post quickly racked up more than 4,000 likes and tens of comments from delighted royal fans, with one branding Fergie the "most real and authentic member of the royal family".
"She's not afraid to not look perfect, she shows emotion on her face, and she's out there working for causes that have earned her respect! Go Sarah!" one Instagram user wrote.
"Looking beautiful, Sarah! You are lovely inside and out," another said.
Some took the chance to urge the mother of Princess Beatrice and Princess Eugenie to remarry the Duke of York, as reconciliation rumours continue to engulf the former spouses who famously refer to themselves as the world's "happiest unmarried couple".
"I'm still waiting for the Prince and Sarah to remarry. Y'all are perfect for each other," an American fan wrote.
And Buckingham Palace sources tell New Idea the royal 'soulmates' are "pulling out all the stops to make things official" before Fergie's 60th birthday on October 15.
The Yorks set tongues wagging by sitting side by side to watch their youngest daughter Princess Eugenie marry her longtime love Jack Brooksbank at St George's Chapel in Windsor Castle last October, and sparked further speculation by getting off on a family holiday to Bahrain with Princess Beatrice and her new boyfriend Edo Mapelli Mozzi in April this year.
The Duchess - who, despite her and Andrew's 1996 divorce, is said to remain a favourite of Queen Elizabeth II - has attended a litany of official royal family events including the London wedding of Lady Gabriella Windsor and the preview of Trooping the Colour in June.
Since the birth of Prince William and Kate Middleton's third child, Prince Louis, in April 2018, Fergie and Andrew no longer need the Queen's permission to wed with the Duke now seventh in line to the British throne, per the Royal Marriages Act of 1772.