Luke Donald (67) and overnight co-leader Mark Hensby (73) tied for third place at 7-under-par 277, while Stuart Appleby (72) and Geoff Ogilvy (73) shared fifth at minus-6.
Tiger Woods, a three-time winner who titled here last year, never mounted a final-round charge. He shot an even-par 71 and tied for seventh place with Davis Love III (68), Carl Petterson (69) and Jim Furyk (71). The group came in at minus-5.
'I just couldn't get it close,' said Woods, who nearly missed his first cut in 126 starts on Friday. 'I needed to give myself a lot of looks and I did. Unfortunately they were from 20, 30 feet on every hole. You know, that's fine if you're leading the tournament, but I wasn't leading the tournament.'
With Woods in striking distance of the lead, Ames used steady play on Sunday to secure his first PGA Tour title.
Ames shared the 54-hole lead with Hensby and ran into trouble early in his round. He bogeyed the third hole, but no other player on the leaderboard was going low due to swirling wind and the tough conditions at Cog Hill.
Ames got into a tie for first with a spectacular iron shot at the par-3 12th. He ran home the 6-foot birdie putt to join Lowery in the lead, but trouble loomed for Lowery.
Lowery hit an awful drive short and in the left rough at the par-4 13th and walked off with a double bogey. He fell two behind the pace Ames was setting.
At the par-5 15th, Ames reached the green in two. He two-putted from close to 50 feet for birdie and a three-shot lead at 10 under par. Ames' margin look threatened at the 16th hole. He drove in the first cut of rough and missed the green with his second. Ames chipped to 10 feet to stay at minus-10.
Lowery made it interesting with a birdie at the 17th hole. He now trailed by two, but drove into a fairway bunker at the last and looked like he might fall into a tie for second. Lowery sank a 7-foot par save to polish off a round of 1-under 70.
Lowery also earned a spot in the British Open in two weeks thanks to his high finish in this tournament.
'I wasn't thinking about the British Open thing at all,' admitted Lowery. 'I knew if I made that I'd be sole second, so I was happy to knock that putt in. I didn't try to qualify at Congressional and I wasn't planning on going. I had already committed to tournaments over here. I'll probably withdraw out of some of these tournaments and go play the British Open.'
Ames stood on the 18th green with another birdie opportunity. His 35-footer stopped a foot from the hole but no matter. Ames secured the victory and took the $864,000 first-place check.
'For me, it was emotional,' said Ames, a two-time winner on the European Tour. 'I had to hold tears back and stop thinking ahead of myself. Overall it was very emotional for me.'
Ames, the lone player on the PGA Tour from Trinidad & Tobago, picked up PGA Tour victory No. 1 at the age of 40. The last player to collect his first tour win at the age of 40 or older was Brad Bryant at the 1995 event at Disney World.
'I'm not given the recognition of my career, on what I've done, because the sports in Trinidad are cricket and also soccer,' said Ames. 'I think a lot of things are going to change.'