Marshall, a fourth-year Tour veteran, received a putting tip from his father that helped him navigate the front nine in just 29 strokes.
"What really seems to help me go low is my putter. I got a lesson from my dad last night and all of a sudden it started clicking," Marshall said. "The greens here are the best I've ever seen in an afternoon round."
Marshall also feels healthy after overcoming nagging inflammation in his left shoulder that had been bothering him since last summer.
"I really didn't think about a score," he said. "I was injured most of the winter and this is really my first tournament, so I didn't have many expectations.
"I went out and played and all of a sudden putts started going in, and before I knew it I was about 7-under."
Joe Panzeri was one stroke behind Marshall after opening at 6-under 64.
"I think playing the course and the event the last couple of years really helps," Panzeri said. "Feeling comfortable out here is really important."
Riley Wheeldon of Comox, B.C., was the top Canadian, tied for third at 5-under 65.
The tour's leading money winner at season's end will be fully exempt on the Web.com Tour for 2014, with the next four players receiving highly placed status.
"The incentive on this Tour is a lot bigger and I think everyone's really motivated," Wheeldon said.
Nick Taylor of Abbotsford, B.C., opened at 3-under 67, while Toronto's Albin Choi carded 2-under 68 in his first ever professional round.
Matt Makinson of Coquitlam, B.C., had the first hole-in-one in PGA Tour Canada history, holing an eight iron on the 171-yard 17th hole.