Horschel became a two-time winner at TPC Louisiana. He captured his maiden PGA Tour triumph at the 2013 Zurich Classic when it was a traditional, every-man-for-himself event.
Horschel and Piercy surged into the lead with birdies on the 10th and 11th holes. They followed with seven pars to hold off Jason Dufner and Pat Perez by a stroke. Dufner had a 14-foot putt for the tie on 18 but left it a foot short.
This was the second year since the Zurich's switch to a two-player team format.
The victory was Horschel's fifth and Piercy's fourth. They each earned $1.04 million and 400 FedEx Cup points.
The result did not count toward the world ranking.
A year ago, the Zurich format called for best-ball play in the second and final rounds, with alternate-shot on the first and third. But organizers decided this year to switch that around so the final round would have the alternate-shot format, with one player hitting the tee shots on even-numbered holes and the other on odd numbers. That set the stage for substantial moves up and down the leaderboard; players had no choice but to play the lies their teammates left them on the previous shot.
Horschel and Piercy began the day three shots back but immediately surged into contention with birdies on the first two holes.
Piercy's 146-yard approach set up Horschel's 5-foot birdie putt on the opening hole. On the par-5 second hole, Piercy chipped to 4 feet to set up another birdie putt for Horschel.
Horschel returned the favor on the par-5 seventh with a chip to 3 feet and did even better on the 10th, dropping a 148-yard approach shot a foot from the hole.
Horschel's 88-foot wedge out of a green-side bunker stopped less than 2 feet from the hole on the par-5 11th to set up his team's final birdie -- and that was just enough.
Heading into the final round, Kevin Kisner and Scott Brown, the 2017 runners-up in a playoff, topped a crowded leaderboard that featured 13 teams within four shots of the lead. By the time the top five teams had reached the back nine, they were all within two shots of one another.
This time, Kisner and Brown faded on the back nine, with three bogeys and one double-bogey on 16 after going into the water. Their final-round 77 dropped them all the way back to a tie for 15th at 15 under.
Garnett and Hadley began the day two shots off the lead and vaulted to the top of the leaderboard with four birdies on the front nine, but they bogeyed the par-4 13th when they struggled with a massive fairway bunker on the Pete Dye-designed course and bogeyed the par-3 14th and par-3 17th.