The Scots, making their first appearance in the tournament since 1998, earned their first World Cup victory since 1990, and started their tournament by going top of their group.
Group favourites Brazil and Morocco played to a 1-1 draw earlier on Saturday (Sunday AEST), putting Scotland two points clear in Group C.
Haiti, whose only other World Cup appearance was in West Germany in 1974, are still in search of their first World Cup point.
McGinn's goal came off a rebound from Che Adams' miss in the box that bounced off Placide and into open space. McGinn's shot ricocheted off a defender from 13 yards out.
Scotland's Tartan Army supporters were out in mass, creating a wave of red inside a filled in Gillette Stadium, located about 30 miles outside of Boston.
Scotland came close to a goal in the 17th minute when captain Scott McTominay got loose and fired a shot that clipped the top of the post.
Haiti had their best opportunities in the second half. In the 74th minute, Ruben Providence sent a cross in to Wilson Isidor, but the forward came up empty on his attempt to direct the ball in.
Then in the 84th, Frantzdy Pierrot's header went a wide of the left side.
Scotland stay in town to play Morocco at Gillette Stadium on Friday. Haiti will next face Brazil on Friday in Philadelphia.
13°C