And he reflected on the joy of becoming a father to Archie and Lilibet ahead of the milestone.
The Duke of Sussex said: "I was anxious about 30, I'm excited about 40.
"Becoming a father of two incredibly kind and funny kids has given me a fresh perspective on life, as well as sharpening my focus in all my work.
"Being a dad is one of life's greatest joys and has only made me more driven and more committed to making this world a better place.
While he will spend the day with his family at home in California, Harry's father, King Charles, will call his second son to wish him well. But the Mirror understands his estranged brother Prince William will not mark the day in any way, highlighting the gaping chasm that still exists between them.
Harry saw his brother two weeks ago at a memorial service for their uncle Lord Robert Fellowes - but the pair never once spoke, despite being only a few feet away from each other.
A source told the Mirror: "What a difference the years have brought.
"When Harry turned 30, it looked like nothing could come between him and his brother. Together, they were set to become the new face of the royals, injecting a much needed modernisation of The Firm.
"But now, William and Harry don't even speak.
"Before, they always celebrated major milestones together but there will be no transatlantic call between to the two on Harry's big day." On Harry's 30th, William attended a raucous black-tie party at Clarence House. Then, young Harry longed more than anything for a dog and a house in the country.
At the time, the Prince had split from his on-off girlfriend Cressida Bonas, and was known to want to find a partner he could settle down with.
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Denne historien er fra September 14, 2024-utgaven av Daily Record.
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