DALLAS, May 18, 2014 (AFP) - Todd, who led after 36 holes and shared the lead after three rounds, finished 72 holes on 14-under 266 to take the $1.242 million top prize at the TPC Four Seasons.
The breakthrough triumph came in Todd's 77th career US PGA start and booked the Atlanta native a spot in next year's Masters, just down the road from his home town.
"It feels amazing," Todd said. "It's a big monkey off my back. I've been through a lot of ups and downs but I'm pretty happy right now."
Weir, the 2003 Masters champion, was second on 268 after shooting 67 Sunday.
Australian Marc Leishman and American Charles Howell shared third on 270 with Americans Boo Weekley and James Hahn another stroke adrift.
Todd, who never before cracked the top-five in a PGA event, had previous career-best efforts of a share of six at this year's Texas Open and Humana challenge.
Todd birdied the par-3 second and fifth holes, the first of them from 34 feet and the other from 20 feet closer. He added back-to-back birdie putts at the ninth and 10th, the latter from 24 feet.
Weir, a 44-year-old left-hander, opened with back-to-back birdies and added two more at the fourth and par-3 fifth.
But he stumbled with bogeys at the sixth and ninth as Todd surged ahead. Weir sank a 21-foot birdie putt at the par-3 13th but twice found the rough at 15 on his way to a bogey.
Weir birdied the par-5 15th to pull within two shots of Todd and reached the clubhouse two adrift.
But Todd, 28, sank a 14-foot par putt at 17 to stay two in front.
"That was huge," Todd said. "I didn't know where I stood. It was a great putt. I rammed it in there."
Todd closed with a four-foot par putt to complete the triumph.
Leading scores after Sunday's final round of the $6.9 million US PGA Tour Byron Nelson Championship (USA unless noted, par-70):
266 - Brendon Todd 68-64-68-66
268 - Mike Weir (CAN) 68-66-67-67
270 - Charles Howell 68-66-69-67, Marc Leishman (AUS) 66-68-68-68
271 - Boo Weekley 67-68-68-68, James Hahn 71-65-65-70
272 - Dustin Johnson 69-69-68-66, Matt Kuchar 69-67-68-68, Graham DeLaet (CAN) 68-66-68-70, Gary Woodland 68-67-66-71
274 - John Senden (AUS) 70-70-68-66, Charlie Beljan 72-65-70-67, Shawn Stefani 74-66-67-67, Charl Schwartzel (RSA) 73-67-67-67, Louis Oosthuizen (RSA) 68-68-64-74
275 - Kevin Kisner 69-70-70-66, Paul Casey (ENG) 71-63-73-68, Tyrone Van Aswegan (RSA) 67-68-72-68, Billy Hurley 70-69-68-68, John Huh 67-71-66-71, Morgan Hoffmann 68-66-68-73
276 - Tim Herron 68-66-74-68, Tim Wilkinson (NZL) 66-71-71-68, Scottie Scheffler 71-68-69-68, Ryan Palmer 67-68-71-70, Andres Romero (ARG) 71-66-69-70, Greg Chalmers (AUS) 71-67-65-73, Padraig Harrington (IRL) 68-68-66-74