From my own notes (there may be other occasions):
May 9, 1937 - Troops of the Canadian Coronation Contingent are the first Dominion troops to stand sentry duty at St. James and Buckingham palaces.
April 17, 1940 - The guard at Buckingham Palace is provided by the Royal 22e Regiment, the first non-english speaking troops of any country to perform this duty.
April 30, 1998 - PPCLI perform Changing of the Guard in London, the first Canadian troops honoured with ceremonial duties at the palace in a generation.
September 3, 2000 - Soldiers of the Second Battalion, The Royal Canadian Regiment, begin a month of guard duties at Buckingham Palace.
September 8, 2000 - Soldiers from Lord Strathcona's Horse (Royal Canadians) form the Queen's Life Guard mounted troop at Buckingham Palace â “ the first time soldiers from outside Britain have been given this honour.