The royal visitors then ventured off to separate parts of the show, which started on Thursday and runs until Saturday.
Prince William spoke to a number of people and made a visit to the site’s mental health hub, which is operating for the first time this year.
Katie Mutton from Pentreath – one of the eight charities working at the hub – said it was an “absolute thrill” to meet the prince.
Ms Mutton added: “We all know mental health means so a lot to him and he does an awful lot of work for that, so it’s a real honour and privilege to be here to meet him.”
Prince William said the charities found “a way to help each other out, the synergy will make a greater impact”.
Serena Jolly from Sunrise Cornwall said afterwards: “Suicide isn’t a dirty word, we have to be able to talk about it.”
The prince also spent a lot of time speaking to visitors to the show including Holly Whitehead, who had travelled down with her family from Derbyshire.
“He was so lovely,” Ms Whitehead said.
“We met King Charles a couple of years ago, so we’re very, very lucky.”
Prince William also took interest to a robot dog-like device, being developed at the University of Plymouth, which can assess the soil quality of fields.
He asked “what will the sheep make of that?” and said “dogs hate it, I’m sure”.