King Philippe of Belgium, and his wife Queen Mathilde, were also present in remembering those who died in battle.
A wreath-laying ceremony took place before a poignant rendition of the Last Post was played by buglers underneath the gate, the spot where thousands of British soldiers passed through in 1917 to reach the battlefield.
Catherine, dressed in a cream Alexander McQueen dress, stood between William, who wore a blue suit with medals, and a more sombrely dressed Prime Minister Theresa May.
Almost 4,000 relatives of the soldiers attended the ceremony at Tyne Cot cemetery in Ypres, Belgium, today alongside British Prime Minister Theresa May and Prince William and wife Kate. Joining them will be 4,000 descendants of those who fought, who won tickets from a government run ballot.
The area is the largest Commonwealth burial ground in the world, with 11,971 servicemen laid to rest and remembered there, 8,373 of whom remain unidentified.
The royals and Mrs May later attended a brief reception at Ypres Cloth Hall, meeting relatives of those killed in the battle. As of Sunday the 30 of July, it has been played 30,752 times since 1928.
The British offensive was staged near Ypres from July 31 to November 10 1917, over fields of liquid mud.
England's royal family traveled to Belgium Monday to commemorate the 100th anniversary of the battle of Passchendaele, or "The Battle of Mud", which caused more than 500,000 casualties for the Allied and Central powers.
'This was a battle which touched communities across Europe and it is a privilege to be here in Belgium to stand as friends with the representatives of all the countries who took part in the battle - friends who continue to be strong allies'.
About 275,000 Allied troops and 220,000 Germans died.