Earnhardt finished second, his best career showing at a track where his late father won nine times. Jimmie Johnson was third, last year's Southern 500 winner Matt Kenseth fourth and Greg Biffle fifth.
Harvick led 239 of the 374 laps, seven more than planned, and became the first two-time winner in the first eight races of the season. It also gave a bit of relief to those winners under the new 16-team Chase for the Sprint Cup championship that they wouldn't have to jostle with 26 winners there are 26 races before the final 10 playoff events to try and make it in.
Harvick had just three top-fives and six top-10 finishes in 17 career Darlington races before this. He hadn't finished better than fifth in his last 10 tries, but clearly had the strongest machine this time out.
He was cruising along with 10 laps left when Joey Logano's car leaked fluids on the track. Johnson and Earnhardt took just two tires in the pits and snuck out ahead of Harvick, who had taken four tires and restarted fifth.
Earnhardt quickly moved past Johnson for the lead until Denny Hamlin hit the wall for the 10th caution to set up the overtime finish.
Earnhardt was still out front on that restart when Clint Bowyer got too close to Kurt Busch and spun him out Busch made sure to walk up the track and stare down Bowyer to force a second green-white-checkered finish.
This time, Harvick and his fresher tires easily got past Earnhardt and drove on to his second win since joining Stewart-Haas Racing this offseason, adding Darlington to his win in Phoenix this year.
Kyle Busch was sixth and points leader Jeff Gordon seventh. Rookie Kyle Larson was eighth with Tony Stewart and Ryan Newman rounding out the top 10.
But there were plenty of powerhouse drivers who hadn't yet broken through the three Hendrick stalwarts in Gordon, Johnson and Kahne among them and that made for a frantic charge at the end on the quirky, country track.
Harvick was passed at the start by Joey Logano, who led the first 37 laps, then moved back to the front and led for 137 of the next 185 laps.
The worst looking of the seven caution periods was with Paul Menard, who hit the wall yet stayed out, eventually sparking under the passenger side and igniting a small fire in the pits. No one was injured, although Menard's car was towed to the garage for repairs.
"I guess I was just in denial, I guess I didn't realize the right-front tire was down," Menard said.
Menard brought out another caution with 95 laps left, hitting the wall a second time.
The race teams get a break next week for the annual Easter holiday before resuming the season at Richmond International Raceway on April 26. Their next off weekend won't come until July 20, a week before the Brickyard 400 at Indianapolis Motor Speedway.
Sprint Cup standings
Driver Pts behind
1. Jeff Gordon
2. Matt Kenseth -1
3. Carl Edwards -19
4. Dale Earnhardt Jr. -26
5. Kyle Busch -28
6. Jimmie Johnson -27
7. Brad Keselowski -51
8. Joey Logano -52
9. Ryan Newman -61
10. Austin Dillon -62
11. Greg Biffle -70
12. Tony Stewart -73
Note • For Sunday's race results, see Scoreboard on page C14.