Benson, the winner of four of the last five races, joined Mike Skinner (1996 and 2007), Ron Hornaday (1997), Greg Biffle (2000) and Todd Bodine (2005) as the only drivers to win three races in a row in the series' 13-year history.
Darnell held on to finish second. Polesitter Bodine led the first 74 laps and finished third. Jack Sprague was fourth, followed by Hornaday.
Benson, also the 2006 Nashville winner, had a fast truck early, charging from 16th to fourth by lap 30. But his Toyota was loose and he spent much of the first half of the race outside the top 15.
After one early stop, Benson was sent to the end of the lead line after a tire rolled out of his pit stall. That, coupled with an unresponsive set of tires contributed to his troubles.
"It was a tough run," said Benson, the only two-time Nashville winner in the Truck Series. "At the start of the race we were definitely passing people but we had a pit-stop that didn't go quite the way we wanted it to. Those tires were pretty challenging.
"When the truck wasn't quite right I started getting a little irritated. Everything we did wasn't doing anything.
While never completely satisfied with his truck's performance, Benson's fortunes turned at the halfway mark of the 150-lap event as he picked up 10 positions during a yellow-flag pit stop.
"We only took gas and that's what we had to do to get back in the race," Benson said. "Then at the end we took two tires and the other guys didn't take any. I don't think the truck was right, but it looked like a lot of people had problems out there, slipping and sliding. I was one of the few who was able to hold the bottom real tight."
That was evident in the final stages of the race. Benson rejoined the top five with 35 laps to go and was on Darnell's bumper by lap 143. Benson dove to the inside at the stripe on lap 147 before drawing clear and winning by 0.889 seconds.
"Clean air seemed to work tonight and there wasn't much of a top groove, especially in three and four," Darnell said. "I was doing my best to keep Johnny back there, but once he got a little bit of clean air when he got beside me, he was able to make his move. Not taking tires at the end and not making adjustments were the difference."
Benson, a 13-time winner in the series, leads Hornaday by 45 points with 10 races remaining. The series' next stop is Aug. 20 at Bristol, Tenn.
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