Gun salutes were fired across London, with the King's Troop Royal Horse Artilliary firing 41 volleys at Green Park from 12pm.
Immediately afterwards the Band of the Scots Guards performed Happy Birthday in the park.
Military bands also performed a special rendition of the song outside Buckingham Palace during a Changing of the Guards ceremony.
An hour later the Honourable Artillery Company fired a 62-gun salute at the Tower of London.
A new portrait of the King has also been released for mark his 74th birthday.
The photo shows him leaning against an ancient oak tree in Windsor Great Park.
Buckingham Palace also used Charles' birthday to announce he has officially become Ranger of the Park - 70 years after his father, the Duke of Edinburgh, was appointed to the post.
The role of Ranger offers oversight and guidance to the deputy ranger and his team in the day-to-day stewardship of one of the country's oldest landed estates.
Paul Sedgwick, The Crown Estate's managing director, rural and deputy ranger of Windsor Great Park, said: "We are honoured to have His Majesty as Ranger of Windsor Great Park, continuing a long tradition of the Sovereign and members of the Royal Family holding this role.
"Windsor has a wonderful heritage with many precious natural habitats.
"His Majesty's passion and commitment to the natural world will be invaluable as we seek to become a centre of excellence for environmental best practice, preserving and enhancing the Great Park for generations to come."
Birthday messages have been posted online by various members of the Royal Family.
The Prince and Princess of Wales wrote: "Wishing a very happy birthday to His Majesty The King!"
A birthday wish was also posted on the Royal Family's official Twitter account, with a post reading: "Wishing His Majesty The King a very happy birthday today."
The King is said to be celebrating his birthday in private.
Charles became King after the death of the Queen on September 8, and has had a busy period since.
As Prince of Wales, he was sometimes away on official foreign tours during his birthday and would celebrate it overseas with a cake.
Charles spent the day before his 74th birthday leading the nation in honouring its war dead during the annual Remembrance Sunday service at the Cenotaph.
His floral tribute to the fallen left at the memorial in Whitehall featured the words "In memory of the glorious dead. Charles R".
Last week Charles was targeted with four eggs during a visit to unveil a statue in honour of the late Queen at York Minster.
An official photograph has been released to mark the King's 74th birthday - showing him in his new role as ranger of Windsor Great Park.
King Charles III is seen leaning on an ancient oak tree in the park, as he takes on a role once held by his father, Prince Philip.
The photograph, full of autumn sunshine, was taken last week.
The King is expected to mark the day privately, his first birthday since becoming monarch.
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