"I've kept saying the last few weeks, I wasn't far away ... it's so nice to see everything come together, finally," McIlroy said.
"I've worked hard on everything, not just my putting but my ball-striking and iron play."
A host of big names threw their hat into the ring during a thrilling final day, with overnight leader Henrik Stenson carding a spirited 71 to finish fourth at 13 under.
Eight-time Bay Hill winner Tiger Woods delighted enormous crowds when he pulled within one of the lead courtesy of three back-nine birdies, but faded with two bogeys to finish at 10 under.
The 14-time major winner iced his last tournament before the Masters with 69 to earn a second top-five finish in as many weeks.