Fergus' late rally helped him card a 5-under 67 to win by one shot at 13-under-par 203.
'It's been a great week. It was one of those weeks where I hadn't been playing well all year, but I had a bunch of people behind me,' said Fergus, who earned $315,000 for the win. 'I had a couple things that I worked on earlier this week with my putting and it helped.'
Andy Bean was in the clubhouse at 12-under-par 204 after a final-round 65. He shared second place with Mark O'Meara, who played in the final group and closed with a 4-under 68. O'Meara had birdie chances at the final three holes, but was unable to convert.
Joey Sindelar stumbled to a bogey on the 17th to post a 5-under 67. He ended alone in fourth at minus-11.
O'Meara and Sindelar shared the lead for much of the back nine on the Jack Nicklaus Course at Punta Espada Golf Club.
Fergus bogeyed the third, then birdied No. 4. After a bogey on the 10th, he trailed O'Meara by four strokes. Fergus flew into contention with four consecutive birdies from the 12th. That jumped him to 11 under, where he was still a stroke out of the lead.
The 55-year-old Fergus drove into the left rough on the 17th and had 99 yards left. He holed that shot for eagle to leap into the lead at minus-13.
'I hit a good drive and I had it set up perfect,' Fergus said. 'I had a perfect angle and a good number. You hit a good shot and get lucky, that's what happened. It was lucky for me it went in.'
Fergus two-putted for par from about 50 feet at the last to seal the win.
O'Meara, who is still searching for his first Champions Tour title, flew out of the gate with birdies on one and two to take a two-shot lead. He rolled in back-to-back birdies again from the sixth to move three ahead at minus-12.
He stumbled to a bogey on the eighth, then carded six straight pars around the turn. O'Meara two-putted for birdie on the par-5 15th and was tied for the lead at 12 under with Sindelar.
O'Meara could only par the final three holes to share second.
Bean birdied the first, then dropped in two straight birdie tries from the fifth. Birdies on 10 and 12 moved Bean to 10 under. He sank an 8-footer for birdie on 17 and rolled in a 25-foot birdie effort at the last to finish at 12 under.
'I made a couple of good putts earlier in the round, especially on the back side and didn't birdie the par-5 (15), but when you birdie the last two holes you can't be upset about anything,' Bean said.