Johnson was tied for the lead after a birdie on No. 16, then drove out of bounds and hit the lip of a fairway bunker en route to a triple-bogey 7 on the par-4 17th. He finished with a 70.
"I was playing really well," Johnson said. "Really confident, swinging the driver really good. So you know, it's a driver hole for me, and I just blocked it a little bit. Made a poor swing. ... Not too happy, but I felt really good with my golf game. ... It's nothing, nothing to worry about. I'll go get them next weekend."
McGirt had a 68, and Kuchar and Bohn shot 71.
David Hearn was the top Canadian, shooting a 73 to tie for 44th at 4 under.
"It wasn't the finish that I was really looking for. I had a nice chance today," said Hearn, a playoff loser in the John Deere Classic. "I'll take away some positives from the week. I appreciate the support from everyone here this week and it always feels good to play at home."
Mike Weir tied for 49th at 3 under after a 72.
"I'm playing fine. Just missing too many short putts," Weir said. "I missed a number the last couple days inside 6, 7 feet, just missing way too many of those. You need those to keep your round going sometimes and I just didn't capitalize when I had opportunities."
Pat Fletcher, born in England, was the last Canadian winner, taking the 1954 event at Point Grey in Vancouver. Carl Keffer is the only Canadian-born champion, winning in 1909 and 1914. Albert Murray, a Canadian also born in England, won in 1908 and 1913.
The 2014 tournament will be played at Royal Montreal.