Prince William and his wife Catherine were to throw a private bash at their Kensington Palace home in London to mark the occasion, with royalty and close friends in attendance.
To mark the prince's birthday they released two new photographs of the family, which show toddler George fascinated by butterflies at the Natural History Museum in the British capital.
In a short message accompanying the pictures, William and Kate thanked the public for their generosity towards the little prince over the last 12 months.
This photo taken Wednesday, July 2, 2014 shows Britain's Prince William and Kate Duchess of Cambridge and the Prince during a visit to the Sensational Butterflies exhibition at the Natural History Museum, London. (AP Photo)
"We would like to take this opportunity on George's first birthday to thank everyone over the last year, wherever we have met them, both at home and overseas, for their warm and generous good wishes to George and our family," they said.
In one of the new pictures released for his birthday, George is looking at a blue morpho butterfly that has landed on the back of his father's right hand.
The youngster, sitting on a bench with his parents, appears about to pick up the insect.
In the other picture, William, Kate and their son are transfixed by something above their heads.
The photographer, John Stillwell, said George, who is third in line to the throne, was "very lively and very sure of himself and confident -- a very determined young boy".
The shots were taken a few weeks ago. George is pictured wearing a pair of dungaree shorts, over a polo neck t-shirt, shoes and socks..
For the party, the London Evening Standard newspaper said Kate has baked George's cake herself.
"The family will mark the birthday privately at Kensington Palace with close family and friends," a royal spokeswoman told AFP.
Newspapers said great-grandmother Queen Elizabeth II would join the party, though her husband Prince Philip is less likely to turn up as he has engagements to attend in Greenwich.
Meanwhile grandfather Prince Charles and his wife Camilla are in Scotland on their annual summer visit.
William and Kate, the Duke and Duchess of Cambridge, are determined to give George as much of a normal upbringing as they can, keeping him largely out of the spotlight.
But the cherubic royal has charmed onlookers during his few appearances in the public eye, particularly during the tour of Australia and New Zealand in April.
And his arrival on the scene has given the royal family a major feel-good factor boost.
"Prince George's outings are a reminder of what makes the monarchy so enduringly popular -- its humanity," said The Daily Telegraph newspaper.
"The lives of the royal family offer benchmarks to our own. They grow up along with us, experience joy, are touched by sorrow, and confirm that life is all the more pleasant when shared as part of the national community."
His Royal Highness Prince George Alexander Louis of Cambridge was born at 4:24pm (1524 GMT) at London's St. Mary's Hospital on July 22, 2013, weighing eight pounds and six ounces (3.8 kilogrammes).
William and Kate have been hands-on in bringing up their son, bathing him and putting the little prince to bed.
They have revealed that George was a loud baby and cried a lot in the early days because he was always hungry.
To celebrate George's milestone, the Royal Mint issued a silver ?5 coin, called a crown, with a design last struck for Queen Elizabeth II's coronation in 1953.
"The choice of a silver coin is significant as crossing the precious metal across a baby's palm is a way to wish them wealth and good health throughout their life, whilst the ?5 coin is a favourite for royal celebrations," said Shane Bissett, the mint's director of commemorative coins.
Just 7,500 of the crowns were struck and they are now changing hands for more than 30 times the face value.
"They sold out very quick actually, in a matter of days," a Royal Mint spokeswoman told AFP.