Making his debut in the event, MacIntyre carded three birdies as he bids to back up finishing joint-12th on his Masters debut last week to secure a return to Augusta National next April.
The 24-year-old Scot opened his birdie account with a two-putt gain at the second before bouncing back from dropped shots at the seventh and eight by getting close to the green at the par-4 ninth and hitting his second to three feet.
His only deviance from par on the back nine came at the 13th, where the 2019 European Tour Rookie of the Year rolled in a 30-footer for a birdie. The effort left him tied for 46th at the end of the opening day.
Earlier, Russell Knox had posted a level-par 71, having started bogey-birdie running up a double-bogey 6 at 13th in between birdies at 10th and 15th coming home.
Australian Cameron Smith shot a bogey-free nine-under 62 to lead by a shot from 2009 Open champion Stewart Cink, who carded an eagle and seven birdies.
US PGA champion Collin Morikawa and Englishman Matt Wallace also started on a positive note with a bogey-free 65, with recent WGC Dell Technologies Match Play winner Billy Horschel opening with a 66 along with fellow Americans Charles Howell and Harold Varner.
Johnson signed for six birdies in a topsy-turvy round while European No 1 Lee Westwood dropped three shots in the last four holes, including a double-bogey 7 at the 15th as he had to settle for a 71.