Johnson was on the show promoting his book, "William's Princess: The True Story of the Romance That Will Change the Monarchy." He noted the couple first met as friends and "eventually that friendship turned to love."
If they do decide to tie the knot, it will be "a modern royal marriage," Johnson said.
"Years ago, they would have married princesses from another country or an aristocrat, as was the case with Princess Diana," he said. "Now, this is an ordinary, middle-class girl who seems to match William intellectually as well as emotionally and physically, I presume!"
Johnson predicted an announcement 18 months or two years down the line, noting, "William himself has said he doesn't want to get married until he's 28."