Koepka had three birdies in a four-hole stretch on the front nine to take the lead and never trailed the rest of the way, closing with a 5-under 65 for a three-shot victory in the FedEx St. Jude Invitational.
With his third victory of the season, Koepka wraps up the regular-season points title a week early and claims a $2 million bonus from the Wyndham Rewards program. He is assured to going into the FedEx Cup playoffs as the No. 1 seed.
Koepka finished at 16-under 264 and earned $1.745 million.
McIlroy, in his first final-round pairing with Koepka, didn't make a birdie until the 14th hole on the TPC Southwind and closed with a 71 to tie for fourth.
Webb Simpson had a 64 to finish second. Marc Leishman (67) was another shot back. Tommy Fleetwood (66) and Matthew Fitzpatrick (69) joined McIlroy at 11 under.
Koepka won for the seventh time on the PGA Tour, and he became the sixth player to win a major championship and a World Golf Championship in the same year. He won the PGA Championship for the second straight year in May, and was runner-up in the Masters and U.S. Open. He tied for fourth last week at the British Open.
He also had a pair of top 10s in this WGC when it was at Firestone, and he had a pair of top 3s at the TPC Southwind when it was the St. Jude Classic.
LPGA TOUR
EVIAN-LES-BAINS, France (AP) — Jin Young Ko won her second major title of the season at the Evian Championship and returned to No. 1 in the women's world ranking.
Ko closed with a 4-under 67 in the rain and seized control when Hyo Joo Kim took a triple bogey from the bunker on the 14th hole. Ko won by two shots over Kim (73), American rookie Jennifer Kupcho (66) and Shanshan Feng (68).
Ko won the ANA Inspiration, the LPGA Tour's first major of the year.
Kim, who won the Evian five years ago, had the lead until her tee shot plugged in front of the face of a bunker. Her next shot rolled back into her footprint in the sand, and she three-putted after getting on the green.
Ko finished at 15-under 268 and earned $615,000 to move ahead of U.S. Women's Open champion Jeongeun Lee6 on the LPGA Tour money list. Ko widened her lead in the Race to CME Globe and replaced Sung Hyun Park at No. 1 in the world.
Kupcho, a former NCAA champion at Wake Forest who won the inaugural Augusta National Women's Amateur in April, earned $290,778 for her three-way tie for third.
PGA TOUR CHAMPIONS
LYTHAM ST. ANNES, England (AP) — Bernhard Langer closed with a 4-under 66 in the rain at Royal Lytham & St. Annes to win the Senior British Open and extend his record with 11 senior majors.
Langer, who started the final round three shots behind Paul Broadhurst, ran off six birdies through 14 holes to seize control with a four-shot lead, and a pair of late bogeys didn't cost him. The 61-year-old German finished at 6-under 274 for a record fourth Senior British Open title.
Heavy rain delayed the final round by nearly six hours.
Broadhurst, trying to win this major for the second time in four years, closed with a 71 to finish alone in second, two shots behind.
Retief Goosen of South Africa closed with a 66 to tie for third with American Tim Petrovic (68) at 3-under 277.
In his last appearance at the Senior British Open, three-time champion Tom Watson shot 73 and tied for 64th. The 69-year-old Watson, who won five British Open titles, said Saturday he would no longer player the tournament. In 18 appearances, he never missed the cut.
Langer has won senior majors in six seasons.
PGA TOUR
RENO, Nev. (AP) — Collin Morikawa birdied his last three holes to win the Barracuda Championship, making him the second player this summer to go from college to a PGA Tour winner.
Morikawa, who graduated from Cal last month, closed with seven birdies against no bogeys in the modified Stableford scoring system, giving him 14 points in the final round at Montreaux Golf and Club.
He finished with 47 points for a three-point victory over Troy Merritt, who had the lead on the back nine until the 22-year-old Morikawa made his final charge. Merritt had a chance to win on the par-5 18th with an eagle, but his chip raced 12 feet by the hole and he missed the birdie putt. He finished with seven points in the round.
Morikawa joins Matthew Wolff of Oklahoma State as players who have gone straight from college to their first PGA Tour victory. Wolff won the 3M Open in Minnesota three weeks ago, making an eagle putt on the final hole to beat Morikawa.
He was doing everything right leading up to his big moment at Montreaux. He has made the cut in all six of his starts since his May 18 graduation, including his tie for 35th in the U.S. Open at Pebble Beach. After his runner-up to Wolff in Minnesota, he tied for fourth in the John Deere Classic and then won the Barracuda Championship.
His 14 points translate to a 65, meaning Morikawa has an average final round of 66.83, his worst Sunday score a 69 in the U.S. Open.
While he was already assured of a PGA Tour card for next year with his good results, the victory gives him membership immediately, with his points retroactive. Morikawa is at No. 46 in the FedEx Cup, and he cracked the top 100 in the world ranking. He earned $630,000, giving him $1,672,904 since graduating college just over two months ago.
KORN FERRY TOUR
SPRINGFIELD, Mo. (AP) — Harry Higgs opened with three straight birdies to take the lead and closed with a 5-under 67 for a two-shot victory in the Price Cutter Charity Championship that locked up a PGA Tour card for next year.
Higgs won the money title on the PGA Tour Latinoamerica last year. His victory Sunday at Highland Springs Country Club moved him to No. 4 in points on the Korn Ferry Tour. The top 25 after the regular season, which has two tournaments remaining, earn PGA Tour cards.
Higgs, who played college golf at SMU with Bryson DeChambeau, started the final round one shot behind Grant Hirschman and took the lead with his third straight birdie. Steve Wheatcroft birdied five of his last eight holes for a 67 and tied for second with Andrew Svoboda (68).
Hirschman closed with a 72 and tied for fourth.
OTHER TOURS
Robynn Ree closed with 4-under 67 for a one-shot victory in the CDPHP Open in Albany, New York, and her first Symetra Tour title. Of the top four contenders, Ree was the only player without a bogey on the back nine of Capital Hills at Albany. Yujeong Son (67), Holly Clyburn (68) and Maude-Aimee Leblance (69) tied for second. ... Joey Lane was declared the winner of the Dongguan Open on the PGA Tour Series-China when more rain cut the tournament to 36 holes. About half the field had to finish the second round Sunday before another delay. Lane didn't have to hit a shot in his three-shot victory over Yuwa Kosaihira of Japan. ... Mone Inami closed with a 2-under 70 for a one-shot victory in the Century 21 Ladies Golf Tournament on the Japan LPGA.