McIlroy produced a superb final-round 65 at Hong Kong Golf Club to finish two shots clear of Gregory Havret on 12 under par, and keep alive his hopes of overhauling Luke Donald for The Race to Dubai title at next week's Dubai World Championship.
But having seeing Westwood tear up the Sun City course in South Africa, the 22-year-old felt he had to triumph in Hong Kong to protect his world number two ranking.
"One of the goals going out today was to win this tournament to keep myself in with a shout next week. It's very dependent on what Luke does because he's got such a big lead but if I can somehow get myself into contention next week, you never know," he said.
"Another little bit of motivation was that I woke up this morning and saw Lee Westwood shoot 62 in Sun City and I thought I really needed to win to stay above him in the world rankings. It was nice to be able to do both.
"I had a lot to play for, trying to keep my hopes alive for the Race to Dubai and seeing that I needed to go out and produce a really good score if I wanted to keep my number two ranking."
Having struggled with fatigue the previous two days, a five kilometre run on Sunday morning before his final round seemed to rejuvenate McIlroy as he carded five birdies in a bogey-free round, chipping in from a greenside bunker at the last to win in style.
Having twice finished runner-up in Hong Kong, including a heartbreaking play-off defeat to Lin Wen-tang three years ago, McIlroy was delighted to finally get his hands on the trophy.
"It feels like it was a long time coming. It's fantastic. I've loved the tournament ever since I played my first one in '07, I felt like the last owed me something after what happened in the play-off in a way.
"I had to wait a few years for it to finally happen but to win this tournament is very special," he said.
"No matter how prestigious tournaments are, you always have your favourites and this is definitely one of my favourites and to be able to win it is fantastic. I couldn't be happier."
Holding a one-shot lead over playing partner Havret approaching the 18th, McIlroy drove into the left-hand rough and his second shot landed in the bunker just in front of the green. Havret, meanwhile, had found sand from the tee and was forced to lay-up.
The Frenchman's third shot landed 20 feet from the flag, meaning McIlroy would need to get up and down for par to at least maintain his advantage.
Instead, he holed his bunker shot for a stunning birdie before launching into wild celebrations.
McIlroy insisted he was never worried, even though it appeared Hong Kong misfortune had struck again.
"It was actually a very positive thing that I hit it in the bunker because a couple of tournaments I've hit it into the bunker on the last and was able to win," he said.
"When I won in Dubai, I hit it into the bunker at the last and in Shanghai, I hit it in the bunker in the play-off. So hitting it in the bunker on 18 has actually worked out well for me.
"It was nice to have those positive thoughts. I was walking up drawing on those memories and knew that I usually do okay out of bunkers on the last hole.
"I feel as if I've struggled off the tee all week. I hit a three wood and basically stood up and hit it as hard as I could and usually if you put a good, committed, aggressive swing, it works out in your favour.
"The second shot I thought I'd drawn a decent lie in the rough but just got a little bit of grass caught between the club face and the ball and it didn't quite fly as far as I thought it would.
"I felt with Gregory being where he was with his third shot, if I could just get it up and down it was probably going to be good enough. I just hit a perfect bunker shot and once it landed on the green, it never looked anywhere else and I think you could see how much that meant to me when the ball went in the hole.
"It's the most excited I've maybe ever been on a golf course. It was just incredible to see the ball drop and realise I'd finally won this tournament."