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.
هذه القصة مأخوذة من طبعة September 14, 2024 من Daily Mirror UK.
ابدأ النسخة التجريبية المجانية من Magzter GOLD لمدة 7 أيام للوصول إلى آلاف القصص المتميزة المنسقة وأكثر من 9,000 مجلة وصحيفة.
بالفعل مشترك ? تسجيل الدخول
هذه القصة مأخوذة من طبعة September 14, 2024 من Daily Mirror UK.
ابدأ النسخة التجريبية المجانية من Magzter GOLD لمدة 7 أيام للوصول إلى آلاف القصص المتميزة المنسقة وأكثر من 9,000 مجلة وصحيفة.
بالفعل مشترك? تسجيل الدخول
JACK IN THE CROCKS
Grealish among 'four or five' injury doubts [1 in Carsley squad... despite Pep insisting City star would miss double-header
ISTANBAWL FOR SORRY SPURS
Defensive mishaps and red card make it a nightmare for Big Ange
LANDING OF THE GIANT
Burgess back from Oz to make his mark with Huddersfield
Remote chance of help...
Pop-up free health centres offer aid to the uninsured amid crisis in richest country in the world
It's about time! Nuke test hero finally gets his medal
Pete, 93, is honoured Oyrs on
Timothy Spall: Wine binge & my life-saving epiphany
Star hails miracle cancer survival
Secret Covid document on priority cases
A SECRET document was drawn up during the pandemic, outlining which patients should be prioritised for care if the NHS became overwhelmed.
IF THEY'RE HERE, WE'LL FIND THEM
Search for Dodies goes on in flood-hit Spain
DRUG TRIAL FOR 'CURE' TO LUPUS
Therapy could revolutionise patients' lives
Smash driver who fled to US is jailed
His lawyer denies claims of SAS link