The American went out in 28 to match the lowest nine holes on the 2021 Race to Dubai and, though he dropped two shots early on the back nine, a strong finish moved him to eight under at TPC Southwind in Memphis.
Harris leads by two shots from two-time WGC winner Ian Poulter, who carded a bogey-free opening effort, as well as Mexican Carlos Ortiz and American pair Jim Herman and Matthew Wolff.
Bryson DeChambeau sits on five-under alongside fellow American Scottie Scheffler and Australian Marc Leishman, with a group on four-under comprising Daniel Berger, Sam Burns, Max Homa and Will Zalatoris.
English is a four-time winner on the PGA Tour, including at this event in 2013 and twice this season, but his victory in Memphis came before the tournament had WGC status.
"Birdieing the first four holes is good all the time. It was just one of those rounds where you're hitting it where you're looking,” said English, who played in the 2011 Walker Cup at Royal Aberdeen.
"Everything was working, you just kind of get in the zone. All you're thinking are positive thoughts, you're hitting good shots and hitting good putts so you just try and keep the gas pedal down.
"I'll just keep doing what I'm doing. It's my seventh time playing this course and this tournament so I feel I've got a good strategy.”
Poulter, who is pushing hard to play in another Ryder Cup at Whistling Straits in September, was also pleased with his day’s work.
“Just steady golf, to be honest with you,” said the Englishman. “Today was one of those days where I tried to feel my way in the tournament and obviously I think I did that really nicely.”
Laird sits in a tie for 26th, having carded five birdies in his 68, one better than MacIntyre, who had three birdies, including his final hole.
Open champion Collin Morikawa shot a 67, as did Justin Thomas and Lee Westwood, but Rory McIlroy sits 62nd in the 66-player field after he had to settle for a 74.