The native of Finland closed with a 2-under-par 68, making his four-round total 6-under 274. His closest challengers ended at 4-under 276.
“It's an unbelievable feeling,” Ilonen said after his second European Tour win this season. “If I had to pick one tournament to win, alongside the majors, it would be this one.”
One of the players finishing tied for second was 22-year-old German Martin Kaymer, who ballooned to a 3-over 73 Sunday.
“You never wish for a finish like that for any opponent," Illonen added.
James Kingston, who started the final round tied for the lead with Kaymer, could manage only 4-over 74 and shared seventh at three-under-par 277 with Paul Broadhurst (71) and American Corey Pavin (67).