Prince William also laid the foundation stone to mark construction on the hospital's new Oak Centre for Children and Young People - the same move his mother did in June 1990 for a wing of the Trust's West London hospital.
"I am personally delighted to be with you today to lay the foundation stone of the Oak Centre, 30 years after my mother did the very same thing at The Royal Marsden in Chelsea, commemorating the building of the Chelsea Wing," William said in his speech.
Fans were moved by the Duke of Cambridge's actions and couldn't help but compare him with his late mother who was renowned for her humanitarian work.
"Just like his mother in many ways," one mused.
"Such a good man. His mum would be so proud of him!" noted another.
"Her legacy lives on in HRH. She must be so incredibly proud! HRH’s commitment to his patronages and charities is so admirable," a third wrote.
One royal fan even shared a sweet, personal story of their own meeting with Prince William whilst their mother was being treated at the Royal Marsden.
"My mum was treated there between 03-07, sadly passing away after," they wrote.
"On one occasion William came to the hospital and ended up having tea with my mum, it wasn’t long before she mentioned she had two eligible daughters to which William replied - ‘I’m ever so sorry Margaret but I am courting a wonderful woman named Catherine’. He really made her day."
Prince William and Duchess Catherine visited the hospital together shortly after their 2011 royal wedding.