Tom Sutton-Smith, a retired advertising executive living in Scotland, told Canadian reporters he painted the portrait of the 32-year-old royal on a whim, never thinking she would see it.
The Duke and Duchess were shown the painting while checking out a special art exhibition in Scotland's Strathearn on May 29 and William in particular seemed especially taken with it.
"His exact words were actually, 'Wow! Brilliant! That's going up in my room.' So that was a nice compliment," Sutton-Smith said of William's reaction.
"The duchess herself, she stretched her hands out to hold the painting and held it for quite some time."
Sutton-Smith has since learned the portrait is being mailed to the royal couple's private home.
The painting was a response to Paul Emsley’s official portrait of Kate which was unveiled in early 2013 at the National Portrait Gallery in London to mixed reviews.
According to the BBC, Waldemar Januszczak, art critic for the Sunday Times, said he was not impressed by Emsley’s painting.
"I was disappointed, to be honest. I have been waiting for it, like everybody else, with great expectation because the Duchess of Cambridge is someone who we know likes art and was presumably going to be an enlightened patron," he said.
"But I think she's been let down really by the picture. In the end it's yet another pretty ordinary painting of a royal of the sort that we've been really churning out for the last few hundred years in Britain."
Since the official portrait was revealed, a friend of Sutton-Smith set up a Facebook page calling for alternative paintings of the Duchess which inspired the rookie artist to start his project.
"The official portrait itself was very hyper-realistic and I wanted to do something slightly different — just a little bit more freedom in it and trying to get more expression into her eyes and her mouth," Sutton-Smith said.