The triumph marked Cooks eleventh on the PGA Tour, and his first in three years, but you never would have noticed watching him Sunday.
Cook came from six shots off the pace, held by overnight leader Jerry Kelly, and ended the tournament in style, rattling off eight birdies and an eagle in the process for a total score of 17-under-par 271.
John Cook talks about winning
His performance surprised even himself.
'It was a big surprise,' said Cook, who was coming off a second-round 74 on Saturday. 'I played so badly yesterday I was embarrassed to be out here, to tell you the truth.
Come Sunday afternoon, it wasnt Cook who was embarrassed, but rather Kelly.
The Wisconsin native and former hockey player was going about his business fine-and-dandy until he reached the par-3 16th.
At the time, he had a two-shot lead over Cook, who was already in the clubhouse, but then proceeded to botch not just the hole, but the entire round, when his tee shot missed the green and he flubbed two chip shots, sent the third running by the hole, and then missed from four feet for double-bogey.
As it was, the triple saw Kelly suddenly fall two back with two holes to play.
The 34-year-old just missed a birdie attempt from 8-feet at No. 17, and then nearly set the house on fire at the 18th, when his approach-second at the closing par-4 hole nearly went in for a tying eagle-two.
The tap-in birdie gave him outright second at 16-under, but it wasnt what he really wanted.
'It was one of those things that makes you love and hate the game,' Kelly said of the 16th. 'It wasn't like a choke kind of thing at all. I went through (the ball) and I just happened to go under it. There was more grass than I thought, apparently.
While Kellys problems down the stretch were about as shocking as Cooks win, it was perhaps the play of a certain former four-time All American who took the real headlines.
Twenty-two-year-old Bryce Molder shot 71 on Sunday to finish in solo third at 15-under.
Playing in just his first event as a professional, he might have won the tournament outright had it not been for four bogies on his card in the final round.
Nevertheless, a solid birdie at the closing hole left a fresh impression in everyones mind that he would be back again ' soon.
First-year player Charles Howell III also had another good showing.
The 2000 NCAA champion from Oklahoma State also closed in 71 Sunday to finish at 14-under, and he now has cashed in eight top-15 finishes in this season alone.
Cook gets the win, however.
Of the rationality of the game of golf, the old veteran philosophized:
'I had made only three eagles all year, and I made four in the last two weeks -- three this week, he said.
'Trying to stay competitive in your mid-40s is not easy.'
Full-field scores from the Reno-Tahoe Open