He posted a six-under 136 total to head Englishman Chris Wood (68) by two strokes, with Spanish world number five Jon Rahm (69) and first-round leader Bradley Dredge of Wales (73) three back.
American Justin Thomas (70) joined a group of four trailing by four shots, but last year's champion Tommy Fleetwood missed the cut, the Englishman perhaps weary playing his third week in a row, including a runner-up finish at the U.S. Open.
Kinhult, according to the European Tour, is the first player to take a halfway lead after starting a tournament with four straight bogeys.
Even a bogey at the last did not detract from the quality of 21-year-old Kinhult's performance.
"I got off to a great start and played well from there," said Kinhult, who picked up four strokes in the first four holes, including an eagle at the par-five third.
"I was six-under on the front nine, which is great so, yeah, happy."
World number two Thomas was disappointed not to be closer to the lead given the quality of his performance in his first appearance in the event.
"It was one of the better rounds I've had in a while," he said. "I had total control of my ball, drove it beautifully, hit some really, really quality iron shots.
"The greens are a little bumpy in the afternoon, so that made it difficult to make some putts, but I stayed patient and was able to make a couple."
A double-bogey at the 17th marred an otherwise mistake-free day, a blemish he blamed on a "freaky little wind" switch that carried his ball into a hazard.
Thomas is playing the event ahead of the Ryder Cup at the same venue in three months time, when he will be part of a talent-laden American team to take on Europe.
The French Open is the first of three consecutive $7 million events in the European Tour's Rolex Series.
(Reporting by Andrew Both in Cary, North Carolina, editing by Ed Osmond)
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