Battling against the wind and rain at Royal Portrush, the overnight leader shot a final round of six-under 66 to win by four shots, seeing off Anthony Wall, Fabrizio Zanotti and Rafael Cabrera-Bello, who finished in joint second place.
Earlier, the Welshman had to cope with a late surge from Swede Mikael Lundberg, who gave the front runners a fright after moving up eight places following two birdies and an eagle.
However, after dropping a shot on the fourth, he fell away, leaving Donaldson to hit four birdies and establish a two-shot lead over his playing partner Wall and then finishing off with his best round of the day.
Local hero Rory McIlroy finished on a disappointing 11-under.
"It's a bit surreal, to be honest," Donaldson said
"I didn't look at the leaderboard all day. I just felt really good out there. It's just been a case of keeping going. I knew that what I was doing was all right."