Spieth entered Sunday's final round with a three-stroke lead on Johnson, but Johnson's 66 tied him with him with Spieth to force a playoff. Spieth earned a one-shot lead on Johnson with a birdie on the 14th hole before Johnson answered back with a birdie on No. 15.
Johnson kept the score tied by sinking a 17-foot putt on the final hole, forcing the extra round. Both golfers shot 13-under on the day.
But the drama was over on 18 at Glen Oaks Club in Old Westbury, N.Y.
The first playoff hole began with Spieth hitting a respectable 315-foot drive down the fairway. Johnson opted to challenge himself on the hole, smashing a 341-foot drive over a left-flanking lake, getting closer to the tee. Johnson hit his next shot 95 yards, with a wedge, within four feet of the hole. He drilled the short putt for the birdie.
Spieth's approach shot couldn't stay on the green. He followed the iron approach with a birdie attempt from the fringe, but he couldn't sink it to match Johnson.
Sunday's win was Johnson's second Northern Trust victory and fourth FedExCup Playoff title. Johnson won $1.5 million with Sunday's victory.
The No. 1 ranked golfer in the world now has four wins this season. Spieth is ranked No. 2 in the world.