King, 36, who represented Team GB at the Rio 2016 Olympics, jumped clear as the last to go over Sunday afternoon's show jumping finale to ensure that Rio team-mate Gemma Tattersall had to settle for second on Jalapeno III.
Tattersall was bidding for a brilliant double having won the short-format competition on Quicklook V on Saturday evening and the rider also finished sixth in the main class on Chilli Knight.
Another Brit, Selina Milnes, finished third on Iron IV while the Queen's grand-daughter Zara Tindall sealed a top-ten finish in coming ninth on Class Affair.
Richmond's Katie Magee enjoyed a dream first Bramham as the highest-placed rider from Yorkshire in 12th on Dollarney. Magee also finished 30th in the short-format competition on Enceladus.
Northallerton Olympian Nicola Wilson came fourth in that event on JL Dublin and 24th in the feature on Yacabo BK while Grafton's James Sommerville retired Altaskin Jack before cross-country.
But there was no denying King a first victory at Bramham, one year after finishing fourth on the same horse in the flagship event of 2018.
"Fourth was a bit annoying last year," smiled King.
"My time faults cross country kept us off the podium. I felt like I had slightly let him down but he was only nine and he did a great job and he came here this year with so much more mileage and experienced.
"He just cruised around the cross country and felt fantastic. It's lovely to finally be at the top of the podium here."
Earlier on, Irish eyes were smiling on Cathal Daniels as the 22-year-old took the British Horse Feeds CCI four star long format under-25s class on Sunday on Rioghan Rua.
Britain's Yasmin Ingham led on Sandman 7 approaching the final phase but one fence down dropped the 22-year-old to second with fellow Brit Will Rawlin also putting one rail down on V.I.P Vinnie to ultimately finish third.
In coming second, Ingham still became national under-25s champion, completing the clean sweep having also been under-21s, under-18s and under-16s champion in previous years.
The flagship CCI four-star long format competition concludes on Sunday afternoon.
"I am delighted to be able to win such a big class," he said.
"Fortunately neither of us feel the pressure and she felt really confident.
"She can be opinionated but we had her relaxed today. It’s a big atmosphere here but she felt fresh and sharp.
"Sure we had a few breath holding moments in the round but in the end she jumped clean and careful.”