Harry joined his grandfather Prince Philip, 95, at the Field of Remembrance at Westminster Abbey.
Harry was immediately charmed by one little boy, Harrison Degiorgio-Lewis, whose uncle Lieutenant Aaron Lewis was killed in December 2008. Harrison, six, proudly wore his uncle's campaign medals and beret, and pointed them out to Harry.
The child's grandmother Helen Lewis said: "We remember Aaron every day and miss him every day. It was lovely to speak to Prince Harry about Aaron. A charity set up in his memory is helping veterans that have been injured, both combat injuries and those with post traumatic stress disorder.Our charity has even helped some of the guys that competed in Harry's Invictus Games and we were talking about that."
At the start of the engagement, Prince Harry and Prince Philip were met by the Dean of Westminster, the President of the Royal British Legion Poppy Factory and members of The Royal British Legion.
Prince Philip, a Life Member of the Royal British Legion, and his 32-year-old grandson each laid a cross of remembrance in front of two wooden crosses from the graves of Unknown British Soldiers from the First and Second World Wars.
The Last Post was sounded, followed by a two-minute silence.
Harry and the Duke of Edinburgh also had the chance to meet veterans from past and more recent conflicts.
Prince William's brother spoke to a former Royal Navy man who worked as an assistant to the US Air Force's 344 Bomb Group. Maurice Jones, 87, has kept a plot in the Field of Remembrance in memory of the American dead. Mr Jones said: "He said 'Thank you for your loyalty. Sixty six years - it has to be a record.'"
The tradition of the Field of Remembrance first started in November 1928 when two remembrance tribute crosses were planted. Ever since, the tradition has taken root, with members of the royal family paying their respects each year.
The Queen and her family will spend the rest of the week attending engagements to mark Remembrance 2016.
On Thursday Prince William will visit Willesden New Cemetery and the Kensington Memorial Park.
On Friday Harry will be out again for the Armistice Day service in Staffordshire.
The Queen and senior members of the family will attend the Royal Festival of Remembrance at the Royal Albert Hall on Saturday, followed by the laying of the wreaths at the Cenotaph on Sunday.