Michael Allen carded a 67 to finish alone in second place at 16-under-par 272. Chris Smith took third place at 15-under-par 273 while David Toms and Tom Lehman shared fourth place at 14-under-par 274.
Geiberger began the day knotted with Lehman at 12 under, but quickly made his claim for the tournament his father, Al, won 28 years ago with birdies on each of his first two holes.
'I didn't want to put pressure on myself to try and get a bunch of birdies right off the bat,' said Geiberger. 'I was trying to hit the fairways, hit the greens and give myself as many birdie chances as I could, just stick with my routine.'
Geiberger then birdied the par-3 fourth and moved to 16 under with a birdie at the par-4 seventh. He kept on rolling with a birdie at the par-5 ninth to go out in 31.
The 36-year-old, who was ranked 144th on the PGA Tour money list coming into this event, put his game on cruise control on the inward half at Forest Oaks with pars on each of his next five holes.
Geiberger, who has struggled with injuries, then sealed the deal with a birdie at the par-5 15th to complete a bogey-free round for his first victory since the Greater Hartford Open in 1999.
'I definitely felt like today I said to myself that I belong out here and I felt like, especially in the past couple months, I feel like I belonged out here,' said Geiberger. 'For a while there it does get discouraging. Just flirting with the cut number, week in and week out. So it gets a little tough at times.'
Allen was also looking for a good finish this week to help secure his playing card for 2005, and the runner-up finish will prove valuable for the 45-year-old.
'I was able to go out and make five or six birdies and, hey, Brent beat me,' said Allen. 'More power to him. It was great playing by him. I'm actually thrilled with my round and being where I'm at.'
Allen tallied three birdies over his first nine holes and moved to 15 under with a birdie at the par-3 12th. He put some pressure on Geiberger with a birdie at the par-5 15th, but a bogey at the par-3 17th squandered his chances at a comeback.
He closed with a birdie at the 18th to secure sole possession of second place.
'I'm definitely relieved. I can tell you that,' said Allen. 'I think in an hour I'm going to be really excited.'
Arjun Atwal fired a 66 to take sixth place at 13-under-par 275. Tom Pernice, Jr. followed at 12-under-par 276.