The 25-year-old golfer from Northern Ireland, who last won at the Dubai Championship in November 2012 to cap a remarkable year, had been runner-up in eight of his last 11 tournaments and did not collect a title in 2013.
McIlroy, a two-time major winner who won for the 12th time on the two biggest tours, overtook some other big-name players to win the European
"It's been 18 months since I won on the European Tour and to win the flagship event, I could not have asked for any more," McIlroy said. "I knew coming in here I was playing well. I struggled a little on Friday but played great over the weekend. I was a little fortunate that some of the guys ahead of me made mistakes and I took advantage of it.
"My caddie JP [Fitzgerald] set me the target of 15 under today. I didn't quite get there but 14 under was enough. I really wanted to win before going into the second major of the season and I could not have asked for a better way to prepare."
Shane Lowry of Ireland birdied No. 18 to move into second place, one stroke behind McIlroy, with a final-round 68.
Bjorn, who shot three-over 75, and Donald, who had a 70, tied for third at 12-under 276.
McIlroy announced on Wednesday, a day before the tournament's opening round, that he had called off his engagement to Wozniacki.
"The wedding invitations issued at the weekend made me realize that I wasn't ready for all that marriage entails," he said.