Kang carded a 23-under 261 to win by two strokes over joint runners-up Matt Every and Scott Piercy of the USA and picked up the trophy. Every, who started that 431-yard par 4 tied for the lead, was in the middle of the fairway before what he called "just a bad shot" on his approach that set up a three-putt for bogey.
Every finished with a 66. "That happened this week", Koepka said.
Every opened the rain-delayed third round with a chip-in eagle, then had four birdies in a span of five holes to take the lead from Sung Kang before play was suspended because of darkness Saturday night with that lead group through nine holes.
Kang matched the course record of 61 in cold conditions on Friday to hold the outright lead by four, and after a six hour weather delay halted Saturday's play and continued in to Sunday, his lead remained at three.
Most players had to complete their third rounds before tackling the final round on a marathon final day.
Every chipped in from the fairway, about 90 feet from the cup, on the par-5 first hole and immediately cut Kang's lead in half.
Five birdies and a bogey on the first six holes got Every to 20 under, and a stroke ahead when he rebounded from that bogey at No. 4 with consecutive birdies.
Jordan Spieth, without a win since the 2017 British Open and without a top-20 finish this season, was tied for 16th at 10 under.
His chance looked to have gone when he surrendered a one-shot lead with bogeys on the 15th and 16th, but the 22-year-old Swede recovered to finish a shot ahead of defending champion Eddie Pepperell (66), Matt Wallace (71) and Robert MacIntyre (68).
Every, who started that 431-yard par 4 tied for the lead, was in the middle of the fairway before what he called "just a bad shot" on his approach that set up a three-putt for bogey. "I only slept about three hours", he said.
That bogey put him at 21 under, tied again with Every at the same time Koepka made a short birdie putt at No. 13 to get him within two strokes of the lead.
Said Every, who played all four rounds with Kang, "I watched him for 72 holes".
Sung Kang reacts after winning the Byron Nelson golf tournament on Sunday, May 12, 2019, in Dallas.
But on Sunday at Trinity Forest Golf Club, Kang stayed strong despite a chaotic final day.
Kang got the victory at the Trinity Forest Golf Club in Dallas, which isn't far from his home in Coppell.
Matt Every only needed half a round to go from a four-stroke deficit to a one-shot lead at the Byron Nelson.