This marks the second victory of the year for McCullough, who won the Mexico Senior Classic in February for his first career Senior Tour title. He became the second multiple winner on the Senior Tour this season, joining Larry Nelson, who captured the MasterCard Championship and the Royal Caribbean Classic at the beginning of the season.
Jim Ahern finished alone in third at 7-under, while Jose Maria Canizares and first and second-round leader John Schroeder shared fourth place at minus six.
McCullough and North returned the par-4 18th to begin the playoff. North drove into a bunker and had to lay up while McCullough, who found the fairway off the tee, hit a 4-iron to 40 feet.
After North knocked his third shot with a 9-iron 20 feet from the pin, McCullough rolled his third to within four feet for par. North's par effort rolled over the lip and McCullough drained his for the win.
'I'm anxious to have a chance to win again,' said McCullough, who pocketed $210,000 for the victory. 'I'm happy because I'm continuing to progress as a player. I just continued to try to make birdies in the last few holes. I focused on making birdies every chance I could.'
McCullough got to 10-under after he drilled a 40-foot birdie putt at the 15th in regulation. North matched him at that score at the 17th when he wedged his approach to 25 feet and converted the putt.
McCullough had his problems on the way to playoff, driving into the rough at No. 17 and missing the green from 135 yards out with an 8-iron. He was was able to get up and down for the save, then hit a 5-iron into the green at No. 18 and two-putted for par.
He began the day tied for third, three shots behind Schroeder. McCullough got into red figures early with a 30-footer at the second for birdie. He added an eight-foot putt for birdie at the 5th before he drained a 50-foot birdie bomb at No. 6.
'It seems like I made a lot of lengthy putts today. I got a couple of breaks early and boom, at the sixth hole I was right in it.'
McCullough missed two birdie opportunities at Nos. 7 and 8 when he failed to convert on birdie attempts from 15 and eight feet, respectively. He made birdie at the 11th, and then holed a 25-foot par save at the next hole after a poor second shot.
McCullough sank the birdie at 15 and tallied pars the rest of the way for a spot in the playoff.
North was tied with McCullough before the round began and birdied the second hole. He parred the next five holes before he holed an 18- foot fringe putt for birdie at No. 8.
North got his back nine off to a good start with a 12-foot birdie at the 10th. He missed a 15-foot birdie at No. 12 and then hit a 9-iron to three feet for a birdie at the 13th.
He wedged his approach to 25 feet at No. 17 and ran home the birdie to grab the lead, but McCullough soon matched him at 10-under. Although North struggled in the playoff, his runner-up finish marked his highest Senior Tour showing since joining the circuit last year.
'I thought I made the par putt on the playoff hole,' said North, an analyst for ESPN. 'I didn't hit the drive I needed to in the playoff.
North claimed $123,200 for second place, the largest check of his career. He took home his previous best of $103,000 for winning the 1985 U.S. Open at Oakland Hills.
North, a three-time winner on the PGA Tour, also captured the 1978 U.S. Open at Cherry Hills in Colorado.
'It was great to be in it today. I'm disappointed I didn't win, but I feel I'm going in the right direction.'
Ahern triple-bogeyed the 10th but rebounded a bit with three birdies the rest of the way for an even-par 70. Schroeder could not get it going on Sunday, mixing three bogeys with a birdie for a 72.
Defending champion Gil Morgan posted a 4-under 66 to share sixth place with Nelson, Gary McCord, Tom Wargo, Graham Marsh and Bruce Fleisher.
Full-Field Scores from the Emerald Coast Classic