ATLANTA — Given a head start, Patrick Cantlay proved uncatchable at the season-ending Tour Championship.

Cantlay started the week at 10 under par and reached 21 under, never trailing for 72 consecutive holes to beat Jon Rahm of Spain, the world No. 1, by one stroke at the Tour Championship to win the FedEx Cup at East Lake Golf Club. Cantlay and Rahm dueled over the last three days, as Rahm kept whittling away at his initial four-shot deficit but could never draw even with Cantlay.

Cantlay’s lead was only one stroke after Rahm converted a par after missing an 11-foot putt for birdie on the 17th hole and Cantlay saved bogey from 6 feet. With $15 million and the FedEx Cup on the line, both players took dead aim at the flag on the par-5 18th hole.

Rahm’s approach from 238 yards skipped within an inch of the flagstick and rolled out just over the apron 18 feet past the hole.

Cantlay stepped up and delivered a 6-iron from 218 yards that rolled 11 feet short of the hole — the closest eagle chance of the day. When Rahm’s eagle chip attempt slid past the cup, all Cantlay needed to do was lag it to 6 inches to make birdie and secure his victory.

“It was the longest lead I’ve ever held, but I just tried to stay, day after day, in the present, and I did an amazing job of that this week because the last couple days I made some mistakes I don’t usually make and I was able to really center myself and hit a lot of good shots when I needed to,” Cantlay said.

“It’s such a great honor because it’s all year and I played so consistent all year and just caught fire the last couple of weeks.”

In the 18 events in which Cantlay and Rahm both played this season, Rahm finished ahead of Cantlay in strokes 14 times — including this week. But Rahm’s 14-under effort on the course was not enough to overcome the four-shot cushion Cantlay started with on Thursday. Rahm and Kevin Na, the third-place finisher, shared the lowest gross scores, but neither was enough to completely make up ground on Cantlay. Justin Thomas finished fourth.

Cantlay started Sunday two up on Rahm and quickly extended his lead to three with a birdie on the second hole. But his advantage was down to a skinny stroke as the players made the turn after Cantlay bogeyed the par-3 ninth hole.

Cantlay had a great chance to go back up by two on the 13th, but his 4-foot birdie putt took a 90-degree turn around the lip and stayed out. It was a surprising miss from Cantlay, who set a PGA Tour record for strokes gained putting (14.68) when he made 537 feet of putts, including every one that mattered down the stretch, last week at the BMW Championship in Maryland.

That hiccup didn’t last, as he buried putts of 4, 6 and 6 feet for par, birdie and bogey on 15, 16 and 17 to take his one-shot lead to the par-5 18th tee.

“We had distanced ourselves from the field and it was like a one-on-one match play feel,” Cantlay said.

Cantlay has finally manifested as the force he seemed destined to become on tour. A decade ago as a freshman at U.C.L.A., Cantlay ascended to the top of the amateur and collegiate ranks, winning four tournaments and sweeping all of the prestige awards. He ranked as the No. 1 amateur in the world a record 54 consecutive weeks and 55 overall — a standard that held up until broken by Rahm in 2016 — before opting to forgo his final two years of college eligibility and turn professional in 2012 immediately after claiming the silver medal for low amateur at the Masters.

It seemed a prudent step considering in his first four tour starts as an amateur Cantlay finished no worse than 24th, including a tie for 21st at the 2011 U.S. Open, and set an amateur record for the lowest score on the PGA Tour when he shot 60 at the Travelers Championship.

Cantlay was considered can’t miss.

But neither golf nor life is that simple. A combination of physical and emotional traumas shaped Cantlay’s early development. A stress fracture in his lower back derailed his transition almost from the start and affected his progress for four years, two of them (2015 and 2016) spent entirely out of commission. During that period, his best friend and caddie, Chris Roth, died in his arms after a hit-and-run while crossing the street in Newport Beach, Calif., in 2016.

“I think as tough as the tough times were, they made me who I am,” Cantlay said. “I’m a better person because of it and I thank all the people that really helped get me through that time and get me to the other side of it.”

Since returning to competition in 2017 and reclaiming his card, Cantlay has been a steady presence. He won his first PGA Tour event at the end of 2017 in Las Vegas and has never fallen out of the top 50 in the world since. He has consistently been around the top 10 before last week’s playoff victory in the BMW Championship over Bryson DeChambeau vaulted him to a career-best No. 4.

At the other end of the field, Joaquín Niemann played alone after Brooks Koepka withdrew midround Saturday with an injury. Niemann decided to try to break Na’s record of playing 18 holes in 1 hour 59 minutes during a Tour Championship at East Lake. Running with his caddie, Niemann shot 72 in 1:54 and finished 29th.

“When I was a kid I used to run a lot in high school and like track and field and stuff, but now I hate it,” he said. “I don’t like running. I just did it for fun and it was pretty fun.”