The former world number one was the overnight leader and although his one-shot advantage came under threat, he claimed another of the European Tour's prestigious Rolex Series events to take his total of victories on his home circuit to 25.
The Englishman's 44th professional win also meant he has won European titles in the 1990s, 2000s, 2010s and 2020s.
Fitzpatrick was two off Westwood before the start of the day and an impressive front nine saw him pile on the pressure.
But the 46-year-old kept his cool and continued to putt well to secure a two-shot victory with a final day five under to finish on 19 under overall and make him a winner in four different decades.
"It's been a good week," Westwood said. "I wasn't really paying any attention to what other people were doing.
“I was trying to control me, control my emotions and control what I'm working on in the golf swing. Just managed to do that.
"(I had) a little slip-up at 16. Like I said yesterday, with what I'm working on, if I don't quite do it, I hit a pull, and I hit a pull second shot and pulled the putt actually, as well, but I hit some good shots coming in, and (I am) really just pleased with the way I controlled myself."
Westwood was 63 in the world going into the event, but the first victory by an English player on the 2020 Race to Dubai should move him into the top 30.