Elizabeth, Britain's longest-reigning and longest-lived monarch, was born on April 21, 1926, and spent 70 years on the throne before her death in September 2022 aged 96.
In a televised statement on Tuesday, Charles said his mother had shaped the world around her, and touched the lives of countless people.
"Today, as we mark what would have been my beloved mother's 100th birthday, my family and I pause to reflect on the life and loss of a sovereign who meant so much to us all, and to celebrate anew the many blessings of her memory," he said.
"Millions will remember her for moments of national significance. Many others for a fleeting personal encounter, a smile, a kind word that lifted spirits, or for that marvellous twinkle of the eye when sharing a marmalade sandwich with Paddington Bear in the final months of her life."
Elizabeth was not expected to become queen when she was born, but after her uncle Edward VIII abdicated to marry American divorcee Wallis Simpson, her father became King George VI when she was 10. She then became queen herself in 1952, aged just 25.
To mark the centenary of her birth, the royal family is carrying out a number of engagements. On Monday, Charles and Queen Camilla visited a new exhibition, dedicated to her fashion.
On Tuesday, the King and other royals will visit the British Museum to view the final designs for a national memorial to Elizabeth in London's St James' Park, featuring statues of the queen and her late husband Prince Philip.
Princess Anne will officially open the Queen Elizabeth II Garden in London's Regent's Park and the King will host a reception at Buckingham Palace in the evening.
"Much about the times we now live in, I suspect, may have troubled her deeply, but I take heart from her belief that goodness will always prevail and that a brighter dawn is never far from the horizon," Charles said.
"God bless you, darling mama. You remain forever in our hearts and prayers."
