The American claimed a comfortable three-shot victory, winning the season-ending tournament and the $10million play-offs bonus.
Horschel was pleased with his ability to reach a new level while competing against the best players in the world.
"I was able to rise to the occasion and get the job done," he said.
"And it just gives me so much confidence, such a thrill to accomplish something like this, especially with the guys I was going up against."
Horschel carded a two-under 68 in the final round to finish three shots clear of Jim Furyk and Rory McIlroy.
He said there were no nerves ahead of his round on Sunday, sensing it was meant to be at the East Lake Golf Club.
"It was very faint, but I remember holding up the FedEx Cup trophy," Horschel said.
"As the season went along, I never thought about it, and figured maybe it was just a dream that wasn't real.
"But I thought about it last week after I won. I've thought about it this week a little bit that maybe this is supposed to happen.
"And maybe that's why when I woke up this morning I was calm knowing that this is my chance to win the FedEx Cup trophy."