The statue, commissioned by the brothers in 2017, was unveiled on Thursday afternoon on what would have been Diana’s 60th birthday.
With Diana’s siblings watching, William and Harry pulled away a cloth covering the monument which will forever remind visitors to her former home of Kensington Palace about its most famous resident.
The figure of the princess is surrounded by three children and depicts Diana in the later years of her life.
In a joint statement, the Duke of Cambridge and Duke of Sussex said: "Today, on what would have been our mother's 60th birthday, we remember her love, strength and character - qualities that made her a force for good around the world, changing countless lives for the better.
"Every day, we wish she were still with us, and our hope is that this statue will be seen forever as a symbol of her life and her legacy.
"Thank you to Ian Rank-Broadley, Pip Morrison and their teams for their outstanding work, to the friends and donors who helped make this happen, and to all those around the world who keep our mother's memory alive."
Kensington Palace said the statue aims to reflect Diana’s “warmth, elegance and energy”, while the children represent the “universality and generational impact of the princess’s work”.
It added: “The portrait and style of dress was based on the final period of her life as she gained confidence in her role as an ambassador for humanitarian causes and aims to convey her character and compassion.”
The royal brothers made no speeches during the ceremony, held in the grounds of Kensington Palace, with the statue standing as their testimony to their mother.
‘An icon who touched the lives of people’
Sculptor Ian Rank-Broadley’s artwork was erected in the palace’s Sunken Garden, one of the places Diana loved most at Kensington Palace.
The space has been redesigned during the past two years and features more than 4,000 individual flowers, including forget-me-nots which were adored by the princess.
The artist said: “Diana, Princess of Wales was an icon who touched the lives of people right around the world so it has been a privilege to work alongside Prince William and Prince Harry on this statue which commemorates her life.
“We wanted to capture her warmth and humanity while showcasing the impact she had across generations. I hope that people will enjoy visiting the statue and the Sunken Garden, and taking a moment to remember the princess.”
In front of the statue is a paving stone engraved with an extract inspired by the poem The Measure Of A Man: “These are the units to measure the worth of this woman as a woman regardless of birth. Not ‘What was her station?’ but ‘Had she a heart? How did she play her God-given part?”’
It was William and Harry’s first appearance together since they attended the funeral of their grandfather, the Duke of Edinburgh, in April.
Diana, Princess of Wales's brother and sisters were seen arriving together at the palace for the unveiling of her statue. Earl Spencer, Lady Sarah McCorquodale, and Lady Jane Fellowes walked through the crowds towards the princess's former home.
Covid restrictions had reduced the number of guests to just 13 and most were gathered between the garden’s side entrance and the statue, erected at the end of a large rectangle ornamental pond.
William and Harry chatted to their uncle and aunts when they first arrived, walking into the immaculate garden together ahead of the brief ceremony.
Members of the statue committee who were tasked in 2017 with commissioning and privately raising funds for the creation of the statue stood nearby.
Lady Sarah was on the committee and her five colleagues included Diana’s close friend, Julia Samuel, who is a godparent of Prince George, and John Barnes, chief executive of Historic Royal Palaces, the charity which looks after a number of royal sites.
The committee was chaired by Jamie Lowther-Pinkerton, William and Harry’s former principal private secretary, and it also included Gerry Farrell, director of the Sladmore Gallery, which specialises in sculpture, as well as financier Guy Monson, a former trustee of the royal brothers’ charitable foundation.
Harry and William chatted informally with the guests, who included the Sunken Garden’s designer Pip Morrison, Kensington Palace head gardener Gary James, and Graham Dillamore, deputy head of gardens and estates at Historic Royal Palaces.
Historic Royal Palaces chairman Rupert Gavin also attended.