Tom Sutton-Smith, a retired advertising executive who had lived in Manitoba for over a decade before moving back to Scotland, said he painted the portrait on a whim and never thought the couple would see it someday, never mind want to take it home with them.
"It was an exciting moment and something you never really expect to happen," Sutton-Smith told CBC News on Tuesday from his home just outside of Kinross, near Perth.
William and Kate were presented with the portrait while checking out a special art exhibition in Scotland's Strathearn area on May 29.
"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 said he has learned the painting is being mailed to the royal couple's home.
The idea to paint a portrait of Kate, he said, came after he and other artists were invited to paint "alternative portraits" as a fun response to the controversy surrounding her official portrait.
The official portrait of Kate, painted by Paul Emsley and unveiled in January 2013, drew criticism from some who believed the 32-year-old looked old and serious.
"A friend, on her Facebook [account], set up a group to do the alternative duchess painting because there was lots of controversy about the official one," Sutton-Smith said.
"I think there was a certain feeling that he had made it too serious and too old."
Sutton-Smith said the finished painting "basically went into a box in my studio" until he was approached by the chair of Perthshire Open Studios, which was hosting an arts exhibition in Forteviot to welcome William and Kate.
"She had to give a gift to the duke and duchess when he was visiting here and remembered the painting and said, 'Can I have it?' I was delighted to do that, of course," he said.
The couple are known as the Earl and Countess of Strathearn in Scotland.
As for how he created the portrait, Sutton-Smith said he relied on photographs of Kate but aimed for an image that was "more an impression and something a little bit freer."
"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," he said.