The Scott Dixon-trained winner beat The Winning Lines, ridden by racing broadcaster Aly Vance, and third-placed Gideon Up, the mount of hat designer Emily Baxendale. Twelve ran, although Harrodian Flyer, the mount of magazine editor Victoria Gray, whipped round at the start and lost all chance of victory.
Harding, who was chief executive of phone group Talk Talk until May, was clearly thrilled with her win, a victory that will rival her runner-up spot when riding her own horse, Cool Dawn, in the 1996 Foxhunter Chase at the Cheltenham Festival. Two years later the gelding won the Gold Cup under Andrew Thornton.
Harding, who had finished second in the Magnolia Cup for the past two years, said: "Goodwood does an absolutely fabulous job of this, and it's a privilege and an absolute pleasure to take part.
"I've been saying I had to keep doing it until I won, but retiring now would be very hard - we'll have to see. I've got my two little girls with me - it's the first time they've seen me ride in a race. They are 10 and 11 and called Emma and Becca - they have seen videos of Cool Dawn quite a few times.
Asked how today's win compared to the Gold Cup triumph as an owner, Harding said: "There is nothing like the adrenalin of riding them yourself, and I've often wondered what it would have been like had I kept the ride on the old horse [in the Gold Cup]. It was a complete fairytale to win at Cheltenham and I wouldn't give it up, but if I only had one day left my wish would be to race-ride again.
"We might go to the pub later for a little celebration, nothing more exciting than that."
Many thanks go to Boots and Spurs aka Duck & Dumplings owners Simon & Louise Chappell for letting him compete in the Magnolia Cup, it was a fabulous day for all.