Paul Tapner and Inonothing on their way to Badminton victory.
Described by fellow riders as one of the hardest working in the sport, Tapner rode with focus and determination to hold his overnight lead and take home the £60,000 first prize.
The win also puts Tapner in second place, behind William Fox-Pitt in the HSBC FEI Classics series.
"This means everything to me," said Tapner. "Every second of this year has been focused on this moment.
"As soon as I first started eventing, Badminton has been my aim. It doesn't get much better than this," he said.
The jumping course, designed by Jon Doney, rode exceptionally well, producing 16 clears, and Tapner was given a confidence boost with a clean round on his first ride, Stormhill Michael, on whom he finished 10th.
Germany's Andreas Dibowski rose from fifth to second when jumping clear on Euroridings Butts Leon - frustratingly, it was the pair's third runner-up placing in their last three 4-stars, following Luhmuhlen and Pau last year.
"My horse was foot-perfect across country, but if I'd just been a little faster or had a slightly better dressage, who knows? Maybe I'll win next year!"
Daisy Berkeley (nee Dick) climbed from sixth to third, her best ever 4-star placing, on the 17-year-old Spring Along. "He was awesome," said Berkeley, of her ride, which has won her a team medal at every level at World, European and Olympic level. "He jumped like a super star."
There was, however, disappointment for Mary King who dropped two places to fourth on Imperial Cavalier, who looked strong and hit the first part of the double.
There was also disappointment for Germany's Simone Deitermann, who hit a catastrophic three fences and plummeted to 12th, while James Robinson rolled just one pole to drop from fourth to sixth.
New Zealand's Caroline Powell backed up her ninth placing last year on Lenamore, to finish fifth, the best placed of the Kiwis.
Andrew Nicholson was next in the rankings, 11th and Nereo, just behind Paul Tapner's second ride Stormhill Michael.
Two jumping rails put Joe Meyer and Snip into 17th place, just ahead of compatriot Mark Todd and Grass Valley. Andrew Nicholson's Avebury was 27th, and Bruce Haskell and Kiwi Smog were 50th. Annabel Wigley withdrew Black Drum from the holding box at the final horse inspection.
Irelands' Camilla Speirs, 20, riding the 155cm Portersize Just A Jiff, a horse she has produced from a three-year-old, won the HSBC Training Bursary worth $1000 for finishing best of the 4-star first-timers in 14th place.
Zara Phillips collected a disappointing 24 jump faults and seven time faults to finish in 48th place on Glenbuck.
Paul Tapner, 34, first came to prominence when winning Bramham CCI*** in 2000 on Highpoint, a year after first arriving in Britain from Sydney, Australia. It was the turning point that made him stay, and he is now married (to Georgina) with two children and lives near Swindon, Wiltshire.
Inonothing has a consistent 4-star record, having been ninth and seventh at Burghley CCI**** in 2008 and 2009, 13th at Badminton in 2007 and seventh at Luhmuhlen last year. Tapner will now be hoping this performance is good enough to earn him his first place on the Australian team for the World Equestrian Games in September.
Inonothing is a 14-year-old Thoroughbred by Basildon Bond, owned by Jenny Waaler.
Tapner admitted that Inonothing failed to impress him as a youngster.
"He had some very special behavioural problems back then - in fact his nickname is 'Spinhead'. After I rode him in his first novice competition in 2003, it went so disastrously wrong that I almost phoned up his owner, Jenny Waaler, and told that I didn't want to ride him anymore. But something held me back from making that phone call. And now look where we are."
50 completed the competition
7 withdrew before jumping
16 clear rounds
Tapner is the third Australian to win Badminton in the last four years, following Andrew Hoy (2006) and Lucinda Fredericks (2007).
• Jim Hayter won this year's farrier award.
1 Paul Tapner/Inonothing (AUS) 44.5 + 0.4 + 0 = 44.9
2 Andreas Dibowski/Euroridings Butts Leon (GER) 43.5 + 6 + 0 = 49.5
3 Daisy Berkeley/Spring Along (GBR) 48.0 + 2.4 + 0 = 50.4
4 Mary King/Imperial Cavalier (GBR) 44.8 + 2.4 + 4 = 51.2
5 Caroline Powell/Lenamore (NZL) 50.2 + 1.2 + 0 = 51.4
6 James Robinson/Comanche (GBR) 43.5 + 4.8 + 4 = 52.3
7 Bill Levett/Hippolyte (AUS) 44.5 + 8.8 + 0 = 53.3
8 Nicola Wilson/Opposition Buzz (GBR) 55.3 + 0 + 0 = 53.3
9 Carolyne Ryan-Bell/Rathmoyle King (GBR) 47.0 + 6 + 3 = 56.0
10 Paul Tapner/Stormhill Michael (AUS) 47.7 + 9.6 + 0 = 57.3
HSBC FEI Classics Standings (after 2 of 5 events)
1 William Fox-Pitt (GBR) 15 points
2 Paul Tapner (AUS) 15
3 Phillip Dutton (USA) 12
4 Andreas Dibowski (GER) 12
5 Becky Holder (USA) 10
6 Daisy Berkeley (GBR) 10
7 Mary King (GBR) 8
8 Boyd Martin (USA) 8
9 Caroline Powell (NZL) 6
10 Stephanie Rhodes-Bosch (CAN) 6