Click on photos below for gallery of Prince William and the Queen.
The new father joined the leader of South Korea, President Park Geun-hye, in unveiling a war memorial at Victoria Embankment Gardens in London.
This is the first time that William has acted on behalf of the reigning monarch during a state visit.
CLICK ON PHOTOS FOR GALLERY OF WILLIAM AND THE QUEEN
The handsome prince looked immaculate in a suit and coat, and proudly donned his Remembrance Day poppy at Tuesday's ceremony.
William watched as President Park uncovered the scale model of the Korean War memorial and cut the first turf. The monument is dedicated to the 100,000 British servicemen and women who served in the Cold War conflict, which saw Communist-backed North Korea battle against US-backed South Korea.
The engagement will be William's only involvement in President Park's three-day visit.
Earlier on in the day, the Queen and Prince Philip welcomed the president at Horseguards Parade. The South Korean leader was treated to a display of British pomp and ceremony and met Prime Minister David Cameron.
Queen Elizabeth and her husband rode with President Park in a horse-drawn carriage to Buckingham Palace, while troops welcomed her with a 41-gun salute on the way. The trio admired an exhibition of Korean-related items in the palace's Picture Gallery.
Tuesday's ceremony will no doubt have struck a chord with William, who has dedicated the majority of his career to the armed forces.
Soon after the birth of Prince George, William announced that he would retire from his military career to gradually take on more royal duties.
The second-in-line to the throne will be joined by Kate later this week on Thursday when the new parents will meet London Poppy Appeal staff and volunteers at Kensington Palace.