The pair spoke to veterans and their families beside the annual display of more than 100,000 small wooden crosses, each with a poppy and personal message.
They honour service personnel who have lost their life since World War One.
The Last Post was played before a two-minute silence and the duke and his grandson planted their own crosses.
It was the third time Prince Harry, who who served in Afghanistan, has visited the Field of Remembrance. The Duke, who was in the Royal Navy during World War Two, has been attending since 2003.
The prince spoke to nine-year-old Rhiannon Lewis, from Essex, who attended the event with her family to remember her late uncle, Lt Aaron Lewis, who lost his life in Afghanistan in 2008.
He also chatted with Liam Young, a former corporal, who served with the Light Dragoons in Afghanistan. The 29-year-old, who was also deployed to Iraq, planted a cross for six of his comrades killed in Afghanistan in 2009 and another who died later in a civilian accident.
The first Field of Remembrance, planted by The Poppy Factory, was set up in the grounds of Westminster Abbey in November 1928.
There are now further fields in Cardiff; Lydiard Park, near Wootton Bassett; Belfast; Gateshead; Edinburgh, and Inverness.
The Duke of Edinburgh and Prince Harry have paid tribute to Britain's fallen soldiers as they opened Westminster Abbey's Field of Remembrance.