Victoria Pendleton finished fifth on Pacha Du Polder as Nina Carberry and On The Fringe won the St James's Place Foxhunter Chase at Cheltenham.
The double Olympic cycling champion had her mount at the rear of the field for much of the race but as her rivals tired she moved up.
But Carberry was far too strong with 13-8 favourite On The Fringe recording back-to-back wins in the race.
"It's probably the greatest achievement of my life," said Pendleton.
"I'm overwhelmed - thank you to everyone who has made this possible. I feel honoured."
Nina Carberry was celebrating her sixth Cheltenham Festival career win
The 35-year-old, who won Olympic gold in the sprint at the 2008 Beijing Olympics and the keirin at the 2012 London Games, had never sat on a horse this time last year.
But she has worked hard over the last 12 months, earning her first win at Wincanton on Pacha Du Polder, trained by Paul Nicholls, last month and the decision was made for her to ride at the Festival.
"I can't believe I did it and I got around," she added. "There are things I would like to do again but this horse is a legend. It's the most fun I have ever had."
Trainer Paul Nicholls told BBC Radio 5 live: "It's great for the team that won the race, but for Victoria to finish fifth after all the doubters was superb. We would not have let it happen if we had a doubt.
"She's had a thrill and I think it's good for racing all around the world. She's ridden a winner in everyone's eyes."
BBC Radio 5 live's Luke Harvey: "That was an unbelievable performance. She rode superbly. You saw her confidence increase and she was weaving and scything through the field. That was unbelievable. Walt Disney stuff.
"She was getting a better reception than the winner. To get round at Cheltenham only 12 months after getting on a horse for a first time is amazing."
Owner Andy Stewart is hoping the rules governing amateur licences will be changed to allow Pendleton the chance to take on the Grand National fences in the Foxhunters Chase at Aintree.
He told BBC Radio 5 Live: "All the dissenters said she would fall off at the first, but I've had plenty of my jockeys fall off at the first. She got round nicely and the horse looked after her.
"I just hope the rules will be changed between the Category A and B licences so she can ride him at Aintree.
"She's got to have a couple more rides under Rules or the rules have got to be changed to ride at Aintree."