European Tour rookie Andrew Tampion also bogeyed the 72nd hole and it cost him his first title. Tampion posted a 2-under 69 and shared second place with third-round co-leader Frankie Minoza (71) and Shiv Kapur (67). The group came in at minus-eight.
Jong Yul Suk (73) and Thammanoon Srirot (69) finished tied for fifth place at 5-under-par 279.
Ilonen and Minoza shared the third-round lead as round three was completed Sunday. The players went quickly back out for the final round in threesomes off both the first and 10th holes, but ongoing weather problems resurfaced on Sunday.
Just as the final threesome headed to the back nine, heavy storms and lightning hit the area. Play was stopped for close to two hours, but the final round finished Sunday afternoon.
When play was halted, Ilonen and Minoza were tied for the lead at 9under par. Ilonen got to that point thanks to a pair of birdies and one bogey, while Minoza birdied No. 10 before the horn sounded to join Ilonen atop the leaderboard.
Ilonen took sole possession of the lead with a birdie at the par-3 12th hole, but Minoza tied him one hole later with a birdie of his own at the par-5 13th.
The battle was soon decided between Ilonen and Minoza as Minoza double-bogeyed the par-4 14th, a hole he birdied in each of his last two rounds. Ilonen still had to contend with the Australian Tampion.
Tampion moved up the leaderboard thanks to three birdies and a bogey over his opening nine. He bogeyed the 10th, but got the stroke back with a birdie at the par-3 12th.
Tampion birdied the par-5 17th to close the gap to one as Ilonen continued making pars. Tampion bogeyed the closing hole, leaving Ilonen a two-stroke cushion to play the last.
Turned out he needed it. Ilonen also bogeyed the finishing hole, but it was enough to finally crack the winner's circle.
Alexandre Rocha shot a final-round, 4-under 67 and finished alone in seventh place at 4-under-par 280.
Tony Carolan (72), James Heath (71) and Chapchai Nirat (72) shared eighth place at minus-3.