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.
Prince William and Prince Harry have visited a London memorial garden for their mother on the eve of the 20th anniversary of her death.