Walker began the final round with a six-shot lead, but eventually won her second Ladies' European Tour title on the first hole of a play-off, two months after also needing extra holes to win her maiden title in the Netherlands.
Hull carded a closing 65 to finish alongside Walker on 25 under par, but hit her drive on the first extra hole into trees and missed from five feet for par after a superb approach.
Walker safely made par to seal the win after a closing 71 to add to her three earlier rounds of 64. "It was pretty nerve-wracking," she said. "Charley played brilliantly today because it was slightly tougher conditions as well. I wasn't quite playing as well as I have done the rest of the week but I'm delighted to have hung in there today and got the win.
"I was hoping not for a play-off, I was trying to win it in regulation play but it didn't happen and Charley made a great birdie on 17 to tie for the lead. I was delighted in the play-off that I made a solid par and managed to get it. I'm just living the dream. I'm really happy and enjoying my golf."
Hull added: "I just blocked my tee shot on the play-off hole which was annoying but I hit a good shot in and a good putt. I'm quite happy. It's been a long three weeks and I'm a bit tired now because I've been in contention all three weeks and especially with a major in the middle.
"To finish it off with a second place is not too bad. I'm looking forward to my next competitions after I have a week or so off because I feel like my game is in really good condition. I'll keep doing what I'm doing."