The Princess Of Wales, who died in a Paris car crash in 1997, visited the hospital publicly and privately 17 times and was instrumental in ending the stigma against AIDS patients.
Diana famously changed the perception of AIDS in 1989 when she shook hands and kissed the cheek of an AIDS patient.
Like his beloved mother, Prince Harry, 31, held hands with some of the sickest AIDS patients and spoke to the staff about his mother's impact on the hospital and breaking down barriers around the illness.
Prince Harry also poignantly signed a guest book in the shadow of his late mother in the hospital's "Princess of Wales Board Room." Hanging above where Harry sat, were two framed photographs of his mother.
In one of the photos, Diana is pictured wearing her favorite tiara, the Cambridge Lovers Knot tiara, the same one that Prince Harry's sister-in-law, Kate, the Duchess of Cambridge, wore last week to the diplomatic reception hosted by the Queen.
Harry even met up with a young woman afflicted with the disease who was an HIV-positive 2-year-old when she sat on Diana's lap at the Great Ormond Street Hospital for Children many years ago.
Prince Harry has personally invested himself in seeking to continue his mother's pioneering work on AIDS education and assistance. He formed the charity Sentebale in Lesotho in honor of his late mother.
The charity's name means "forget me not" and helps the vulnerable children of Lesotho who have been affected by the AIDS tragedy. Many are orphans having lost a parent to the disease and others have inherited the disease from their parents. Lesotho is reported to have one of the highest rates of HIV and AIDS in the world.
Prince Harry made his first-ever visit to Mildmay to celebrate the hospital's 150th anniversary. When he was asked to cut an anniversary cake, the fifth-in-line to the British throne joked, "I don't cut cakes much. We normally plant trees. This is something new."
As Harry left the hospital, he was given a photo of Princess Diana kissing an HIV-positive patient named Martin who died just a week after Diana's visit. That photograph is said by many to have forever changed the way the world sees AIDS patients.