Sowards, a 36-year-old assistant pro at Wedgwood Golf and Country Club in suburban Columbus, started the day two strokes back of Coston, who led after the second and third rounds. Sowards built his own lead to five shots midway through the round.
Sowards shot a closing 70 after rounds of 69, 68 and 69 to hold off University of Illinois coach Mike Small by a shot. Small finished with a 69 that included consecutive birdies on holes 13 through 16.
Chip Sullivan of Roanoke, Va., shot a 70 to finish at 280 for third place. Coston tied Tim Fleming of Oklahoma City (70), and Ron Philo Jr. of Fernandina Beach, Fla., (68) for fourth place.
Sowards built his entire schedule around winning the title. He had played Longaberger Golf Club 22 or 23 times before tournament week to gain all the knowledge he would need.
'At the time I had a five-shot lead I thought, man, this is going to be kind of nice - kind of walk up 18 and smile for the cameras,' Sowards said. 'Then Mike started making birdie every hole and made it a little stressful.'
He and his wife, Lynn, are building a house near Columbus, about 40 miles west of Nashport. Sowards said visiting the construction site has served as motivation to win the $60,000 first-place check.
'Every time I drove over there this week, it was like 'Shoot, I need to pay for it somehow,'' he said with a laugh.
Coston, a 48-year-old teaching pro at Semiahmoo Golf Resort in Blaine, Wash., shot a 40 on the front side - and then was assessed a two-stroke penalty. After he bogeyed the par-3 ninth hole, Coston was penalized because his caddie, Dr. Mark Askew, moved a towel covering Sowards' clubs to determine what Sowards had used on the ninth tee.
'Keep the razor blades away from me,' Coston said, trying to force a smile. 'I'm going to have somebody drive me home, man, because if I don't I might not make it.'
The penalty cost Coston $7,500. His final-round 77 left him tied for fourth. Without the two-stroke assessment, he would have finished alone in third.
The top 25 finishers earned spots in next month's PGA Championship at Whistling Straits in Kohler, Wis.
Sowards was 1-over on the first five holes and a shot back of Coston, then birdied the next three holes. His 7-foot uphill putt at No. 6 tied him with Coston.
At the par-5 seventh, Sowards' chip from just short of the green banged off the flag and came to rest on the lip of the cup. His tap-in gave him his first lead.
At the 444-yard eighth, Coston's approach bounced into a back trap and he failed to get up and down while Sowards' wedge shot ended up 5 feet away for the birdie that gave him a three-stroke lead.
Sowards saved par on No. 9 after nearly hitting into a pond in front of the green and Coston, still just two strokes behind, had to settle for a bogey that turned into a triple bogey when the penalty was assessed.
Small's birdie run made things interesting at the finish. He lipped out a 40-foot birdie putt on the 17th hole that would have tied it. Both he and Sowards hit their approaches to the back fringe on the closing hole and both saved par.
'I struggled the first seven holes and hardly hit any good shots. I thought I was out of it,' Small said. 'Then I made the four birdies in a row to get back in it. I thought I made the putt on 17.'