Maggert, like last week's winner Tim Herron, hadn't won since the 1999 season. Maggert's last title came at the WGC-Accenture Match Play Championship.
'It's just great that I can come out here and do this for a living. I'm very fortunate that I can still come out here at the age of 42 and still win.'
Tom Pernice, Jr. closed with a 1-over 71 to end alone in second place at 6-under-par 274. John Cook (71) and Kris Cox (72) shared third at minus- four.
Zach Johnson ended with a 68 to finish alongside Daisuke Maruyama, Briny Baird and Jay Delsing in fifth place at minus-3.
Herron, who shared the third-round lead with Pernice, struggled to a seven- over 77 to tumble into a share of 16th at even-par 280.
Maggert began the final round three strokes behind the leaders. He opened with a birdie on the first, then two-putted for birdie at the par-5 third to move to 6 under.
The three-time winner on the PGA Tour tripped to a bogey on the par-4 fifth. Maggert parred his next five holes.
Maggert dropped his tee shot inside 10 feet at the par-3 11th and drained that putt for birdie to move back to 6 under and a share of the lead. He managed to stay there by sinking a key par putt from 14 feet out at 13.
The three-time American Ryder Cupper took control of the tournament with a 36- foot eagle putt on the par-five 16th. Maggert sank a 33-footer for birdie on 17 to extend his lead, before parring the last to end at minus-9.
'I've struggled a little bit this year. I played a little better than what my position has shown,' Maggert admitted. 'The putter has slowed me down this year, but today the putter got me through.'
Pernice was going for his first win since the 2001 International. He struggled to bogeys on the third and seventh to slip to 5 under.
The 46-year-old managed to get one of those strokes back with a birdie on the par-3 11th. However, Pernice bogeyed the 14th to drop back to 5 under. He birdied the par-5 16th to finish at minus-6.
'Today was a disappointment, obviously. I didn't make any putts,' Pernice said. 'You make one birdie on the final round, you're not going to win out here. Obviously Jeff had great round. I didn't know he went eagle, birdie at 16, 17.
'I made a mistake early at three, then I hit a great shot at four to about five feet for a birdie and missed it. That kind of put me off to a bad start. I battled the rest of the day and controlled the ball pretty good. I didn't have too many bad shots.'
John Senden carded his second straight round of even-par 70 to finish alone in ninth place at 2-under-par 278.
Brent Geiberger (67), Dudley Hart (68), Ryan Palmer (73), Brett Quigley (68), David Toms (71) and Tjaart van der Walt (69) were final players in red figures at minus-1.