The trio traveled to northern France to pay tribute to the 60,000 British soldiers who died in the Battle of the Somme on July 1, 1916.
Red, white and blue confetti fluttered down from a vast war memorial on a hill above a former battlefield and schoolchildren each placed a posy at 600 white gravestones during the moving remembrance ceremony.
The reflections on the sacrifices made 100 years ago had begun for the royal trio earlier in the day when they viewed the two 30-yard-long tapestries depicting the battle, and met British and French schoolchildren at the visitor's center.
The royals then took their place at the gigantic Memorial to the Missing at Thiepval, high above the fields where so many young men fell on that fateful day.
Prince Charles, who attended the service with wife Camilla, Duchess of Cornwall, and actors Jason Isaacs, Joely Richardson and Charles Dance read poignant stories and letters of the fallen at the memorial.
After the sounding of the "Last Post" by buglers, the royals walked among a group of 300 French and 300 British and Irish schoolkids, who stood quietly at the gravestones.
At a poignant vigil on Thursday, William read a piece by the author of the WWI novel Birdsong, Sebastian Faulks. "We lost the flower of a generation; and in the years to come it sometimes seemed that with them a sense of vital optimism had disappeared forever from British life," William said.
"It was in many ways the saddest day in the long story of our nation. Tonight we think of them as they nerved themselves for what lay ahead.
"We acknowledge the failures of European governments, including our own, to prevent the catastrophe of world war."
Prince Harry also spoke at the event, reading the poem "Before Action," by Lieutenant WN Hodgson of the 9th Battalion the Devonshire Regiment, who wrote it before he was killed in action on July 1, 1916.
Before the vigil, the royals climbed to the top of the newly-renovated monument to view the battlefield.