The White Garden, at Kensington Palace, was planted to mark 20 years since Princess Diana died in a car crash.
The Duchess of Cambridge joined the princes on the garden tour.
A spokeswoman for Kensington Palace said: "The engagement will allow the princes to pay tribute to the life and work of their mother."
They met representatives from the causes and charities supported by Diana, including the Royal Marsden and Great Ormond Street hospitals, the National Aids Trust, Centrepoint youth homelessness charity and the Leprosy Mission.
Members of the public have been leaving tributes and flowers at the gates of the palace to mark the anniversary of Diana's death.
The Princess of Wales died on 31 August 1997 in Paris, when William, now the Duke of Cambridge, was 15 and his brother was 12.
The garden at their mother's former home has been inspired by memories of her life, style and image, such as her white "Elvis" Catherine Walker dress.
The White Garden, as it is known, follows a tradition first established at Sissinghurst Castle in Kent, famous for its own white garden created in the 1930s.
Their Royal Highnesses met gardener Sean Harkin who designed the display and Graham Dillamore who knew the princess when he worked there some 30 years ago.
The garden has been open since spring and will continue into September with white roses, lilies, gladioli and cosmos.
It is the fourth London memorial created in tribute to Diana - the others are the Diana Memorial Playground at Kensington Palace, the Diana Memorial Fountain in Hyde Park, and the Diana Memorial Walk at St James's Palace.

The Duke and Duchess of Cambridge's son is the first member of the UK Royal family to have his first birthday marked by a new coin.
The third in line to the throne and future king turns one on 22 July.
The design of the sterling silver piece was approved by his parents; his great-grandmother the Queen; and Chancellor George Osborne.
It bears the heraldic royal arms and the year 2014.
Prince George was photographed at the weekend walking with the help of his mother.
Shane Bissett, director of commemorative coin and bullion at the Royal Mint, said: "We felt there could be no more fitting design to mark the first birthday of our future king than the royal arms - the arms of the monarch - minted in honour of the future heir to the throne.
"The choice of a silver coin is significant too, as the precious metal is closely associated with newborn babies...
"Crossing the palm of a newborn baby with silver is a way to wish them wealth and good health throughout their life, whilst the £5 coin is a favourite for royal celebrations."
The coins will be a limited mintage of 7,500 and will cost £80 each from the Royal Mint.
The heraldic royal arms incorporates the four quarterings of the royal arms, each contained in a shield, as well as a rose, thistle, leek, a sprig of shamrock and a crown.
It was originally created for the Queen and was last used on a coin struck for the British Exhibition in New York in 1960.
George's birth and his christening were also marked with commemorative coins.
Prince William and Prince Harry have visited a London memorial garden for their mother on the eve of the 20th anniversary of her death.
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A commemorative £5 coin is being produced to honour the first birthday of Prince George of Cambridge.