The Australian fired 10 birdies in his first round, including six on the front nine as he reached the turn in 30, had just one blemish with a bogey-five at the sixth.
Leishman was pleased with his start in good conditions on the course, near Chicago in Illinois, and is now looking to continue that going into the weekend as he bids to keep his FedEx Cup ambitions alive.
The 33-year-old, who was seventh in the overall standings going into the third tournament of four in the race for the title and the $10million bonus prize, said: "I hit the ball well and the putter cooperated.
"It's nice when you can get on a run like this and hopefully I can keep it going."
Fellow Australian Jason Day, Charley Hoffman and Jamie Lovemark sit two shots behind him on seven under, while current FedEx Cup leader Jordan Spieth is another shot further back with four other players, including fellow American Rickie Fowler.
Wold number one Dustin Johnson could only manage a level-par 71, while reigning FedEx Cup holder Rory McIlroy was one of only 14 players from the 69-man field to past an over-par score as he finished with a 72.
The Northern Irishman needs a top-four finish in the BMW Championship in order to secure a place in next week's final play-off event, the Tour Championship, and keep alive his hopes of retaining the title he won 12 months ago.