Corinna zu Sayn-Wittgenstein, 56, who has a home in Shropshire, had an affair with the 82-year-old royal between 2004 and 2009.
The Danish-born entrepreneur now leads a quiet life at the 11-bedroom Chyknell Hall, between Claverley and Bridgnorth, and also has a home in London.
But she has found herself at the centre of a storm because of her relationship with the scandal-hit former king.
Juan Carlos, who is married to Queen Sofia, 81, left his homeland in August after it was claimed he allegedly received millions of euros from Saudi Arabia’s late King Abdullah.
Ms zu Sayn-Wittgenstein questioned why “hostility is always channelled towards the woman”.
Speaking to the Telegraph, she compared herself to a modern-day Wallis Simpson, the American socialite who married the Duke of Windsor.
She said: “There is a tendency that when people cannot control a powerful man, they destroy the object of his affection.
“This narrative still survives to this day. You can even see it with Meghan and Harry.
“The hostility always goes to the woman and the poor man is this helpless creature who has been horribly manipulated and it is the woman who has plunged the country into a huge crisis.”
Corinna’s relationship with the then King of Spain was catapulted into the limelight in 2012 after he broke his hip during a safari trip to Botswana, on which she had accompanied him.
She described being under “full-blown surveillance” and a campaign to paint her as “this Wallis Simpson, Lady Macbeth, evil character who’d led this wonderful man astray”.
She accused the Centro Internacional de Inteligencia (CNI), Spanish intelligence, of harassing her. She claimed her Monaco apartment was being watched and that she was being followed.
The former king, who abdicated in 2014, is under investigation by Spain’s supreme court over corruption allegations, which centre on a $100million payment by the late king of Saudi Arabia.
Last month, Juan Carlos announced he was leaving Spain and he is currently residing in Abu Dhabi.
Ms zu Sayn-Wittgenstein has largely kept a low profile since she moved to the Claverley area, sharing her time between there and London.
She is a popular member of the village community. And both she and her son, Prince Alexander, opened a classic car show to raise funds for All Saints’ Church in 2017.