The American had earlier ended the hopes of Frenchman Victor Perez but he faced an in-form man in Scheffler, with the crowd at Austin Country Club firmly behind the University of Texas alumnus.
Against that backdrop, Horschel kept his focus and claimed a first European Tour victory that moves him to the top of the Race to Dubai Rankings presented by Rolex.
Horschel came into this week as the 32nd seed and advanced from Group Four after a play-off with Max Homa.
He had beaten Homa and World Number Four Collin Morikawa in the group stage, while also losing to J.T. Poston, before taking down Kevin Streelman and Tommy Fleetwood on day four.
A 3&2 victory over Perez sent him into the showpiece match and while he only made one birdie in 17 holes against Scheffler, that was enough to hand him the Walter Hagen Cup.
Horschel finished second at the WGC-Workday Championship at The Concession in February and after earning his two best European Tour finishes in back to back starts, he is now aiming to take on Europe in The Ryder Cup later this year.
“I feel like I should have been on Ryder Cup teams before but that’s my fault because I haven’t done what I needed to do to take care of that,” he said.
“If I do happen to make a Ryder Cup team in my career, I’ll be happy because I feel like that’s one thing I can check off of what I have wanted to accomplish in this game.
“I should have at least contended more in some of these WGC events and Majors.”
On the final, he added: “It was one of those days where I didn’t play very good. I didn’t hit it very good, I didn’t feel good with my swing so I was just grinding it out.
“I wish I would have hit it better so I would have had a closer chance of making birdie, obviously it was a tough day. I had two opponents that played really good today but just didn’t make any putts and I did and that’s the difference.”