Brash, riding Hello Forever, was the last competitor to go in an 11-horse jump-off over 160cm high fences at Royal Hospital Chelsea, beating Maher and MTF Madame X by just 0.08 seconds as he pulled out all the stops. Ireland's Denis Lynch was third with All Star 5 at just 1.13 seconds back.
"I'm absolutely delighted," said Brash, who also won the Longines Global Champions League team competition earlier in the afternoon.
"It was a fast jump-off," said Brash, before giving credit to his horse. "He was amazing today. At the last jump, he really tried his best for me."
Smolders extends leads
Harrie Smolders of the Netherlands, riding his stallion Emerald N.O.P., finished fourth in London, meaning he remains at the top of the overall LGCT leaderboard with 263 points with just three events to go.
Smolders is 18 points ahead of Italy's Alberto Zorzi in second place and 43 points clear of Germany's Christian Ahlmann in third.
The 12th-leg of the 15-stage LGCT for the world's top show jumpers, held for the first time at the iconic Royal Hospital Chelsea, was halted for about 15 minutes following a nasty fall for British jumping legend John Whitaker.
Whitaker, celebrating his 62nd birthday, crashed into the eighth fence in the first round of the grand prix when his big gray stallion, Cassinis Chaplin, refused to jump it. Whitaker went over head first, landed flat on his back and then flapped both arms twice before lying on the ground.
The arena went silent as his younger brother and fellow showjumper, Michael, quickly rushed to the scene. Soon after, an ambulance arrived and green sheets went up around as the rider, a six-time Olympian and one of Britain's most decorated riders, lay motionless.
After an agonizing 10 minutes, there was an audible sense of relief in the crowd when an announcement over the tannoy by event organizers told them: "Some really, really good news. John is OK."
Soon after, Whitaker was driven out of the arena in the ambulance to loud applause.
"John Whitaker is having a few checks at the hospital, and was talking with the medics before leaving the showground," LGCT organizers tweeted later on.
Global Champions League
Earlier on Saturday, Brash delighted the crowd by winning the team competition with Ireland's Lynch for Miami Glory, ahead of defending champions Valkenswaard United and Berlin Lions.
Founded by King Charles II over 300 years ago, the Royal Hospital Chelsea houses some 300 veterans who have served in Korea, the Falkland Islands, Cyprus, Northern Ireland and World War II. The pensioners, wearing their trade-mark long red coats and black hats, received a huge cheer when they entered the stadium for the prize giving ceremony of the Global Champions League.
"It was great to get the win, it's been a great team effort," said Lynch, who produced two clear rounds with two different horses in the two-day team competition.
"Delighted to be here, the pressure was on," said Brash after bringing home the team win with a clear round on his 11-year-old gelding Hello Forever.
Valkenswaard United's second place in London meant it is now back at the top of the overall leaderboard with 238 points, three points clear of Hamburg Diamonds and 37 points ahead of Mexico Amigos.
The next leg of the LGCT will be held in Valkenswaard, the Netherlands, next weekend. Rome follows September 21 through 24 before the finals in Doha, Qatar on November 9 through 11.