After matching the lowest final round of the season on the PGA Tour with a 9-under-par 63, Duval was long with his approach at the par-4 18th -- the first playoff hole -- and chipped six feet past the pin.
DiMarco missed a 13-foot birdie putt but watched as Duval's par attempt burned the left side of the cup, then hugged his caddy and broke down in tears.
It has been an emotional season of near-misses for DiMarco, who began the week with a career-high $1.846 million but had been unable to break through on a number of occasions.
DiMarco finished third or better three times in a four-start span this summer. But his inability to win cost him a spot on the United States Ryder Cup team.
Master of the claw putting grip, DiMarco earned the $612,000 top prize to move into 11th on the money list with nearly $2.5 million.
Duval was seeking his second win this season and second straight at Callaway Gardens Resort, where he shot a final-round 65 for a two-shot victory last year.
DiMarco and Duval ended the 72-hole event at 21-under 267. Bob Estes and Neal Lancaster were at 269 with Davis Love III at 270, Jeff Maggert at 271 and Joel Edwards and Sweden's Per-Ulrik Johansson at 272.
Other than Duval, Johansson's brother-in-law, Jesper Parnevik, suffered the biggest disappointment of the day. He struggled to a 71 and ended up a shot behind Kenny Perry for the final spot in the Tour Championship.
Perry cracked 70 for the first time this week, shooting a 66 to tie for 23rd at 279. He will be in the Tour Championship, which invites the top 30 money winners, for the first time since 1996.