The new father's public appearance will most likely be one of his last on the island, where he is stationed as a search and rescue pilot, before his tour of duty ends in September. His wife Kate Middleton and newborn son Prince George remained at Kate's parents' home in Berkshire.
The charming royal started his speech in Welsh to the delight of the crowds who cheered him on. He briefly broke into English, holding up his hand and saying, "Wait for it," then carried on for a few more moments in the local language before switching back.
"My Welsh pronunciation isn't quite great yet but it is slowly getting better I hope," said Prince William.
"Catherine is sorry that she cannot be here today at the county show, but she and George would have loved to have been here. He's pretty loud and of course extremely good looking.
"I have to say that I thought search and rescue duties over Snowdonia were physically and mentally demanding but looking after a three-week old baby is right up there.
"I know that I speak for Catherine when I say I have never in my life known somewhere as beautiful and as welcoming as Anglesey. The views across the Menai straits are undoubtedly among the most stunning in the British Isles.
"I know that both of us will miss it terribly when my search and rescue tour of duty comes to an end next month and we have to move elsewhere.
"From the bottom of my heart, thank you for making my wife and me so welcome when we arrived here, as you do thousands of visitors each year.
"This island had been our first home together, and it will always be an immensely special place for us both.
"Catherine and I look forward to returning again and again over the coming years with our family."
Prince William then ended his speech in Welsh, and was applauded by show-goers as he left the stage.
The new father has been stationed as a pilot at RAF Valley at Anglesey since 2009. It is largely thought that he will return to London to join his regiment, the Household Cavalry's Blues and Royals, and care for his new family there.
Earlier on in the day, Prince William walked around the grounds and chatted to local farmers, show-goers and judges. He had a go at taking part in the falconry display, and also enjoyed watching the gundog display, joking that he "should have brought Lupo for the spaniel show," referring to the family dog.
"He would have just run off and never come back again," the royal told ITV. "It would have been very embarrassing."