Three days after scoring a major win in the $50,000 Horseware Ireland Grand Prix, Katie Dinan still can’t quite believe what she did. “I’m still a little over the moon about the whole thing. It was definitely one of the most exciting things that’s happened to me on or off a horse,” she said.

On Jan. 29, during the second week of the FTI Winter Equestrian Festival in Wellington, Fla., Dinan guided Nougat du Vallet to the top of a 15-horse jump-off filled with some of the biggest names in international show jumping, including Beezie Madden, Lauren Hough, Christine McCrea, Nick Skelton, Kent Farrington and Cian O’Connor. It’s a big deal for any rider, let alone an 18-year-old newbie to the big grand prix ranks. And she was riding Nougat du Vallet in the big leagues for the first time. The win was a bit of a coming of age for Dinan, who is in her first year out of the juniors.

Last summer, Dinan, of New York City, earned individual gold at the 2011 North American Young Riders Championships (Ky.) aboard Vancouver and started making her name known in the grand prix classes. “I think that it’s definitely a major transition when you go from competing against kids your own age to people who have been doing this for longer than you’ve been alive,” Dinan said. She got good grand prix mileage at Spruce Meadows and the indoors shows last year, and started her 2012 by participating in the George Morris Horsemastership Training Session in Wellington on Jan. 3-7.

Dinan thoroughly enjoyed the Training Session, in which selected riders care for their own horses. Dinan even plaited her horse’s mane for the mock Nations Cup on the last day of the session. “It was nice because I knew how to do everything, but it was the first time I’d done everything for one horse all week. I think there’s definitely a sense of pride when you bring your horse out for the day and no one else has touched them. It was wonderful to have the whole week concentrating on just me and my horse,” she said. The session got Dinan thinking independently—a skill that would become essential as soon as WEF started.

Dinan has ridden with McLain Ward for a year, and he’s been essential to her grand prix progress. But Ward broke his kneecap in a fall on Jan. 14. He wasn’t able to attend the horse show at all during the second week of WEF, so Dinan was on her own. “I kept him up to date on what I was doing, and we videoed all my rounds, and I brought them all to him, and we watched them,” she said. Even without Ward, Dinan pulled off a second-placed finish in the Wellington Equestrian Realty Grand Prix on Jan. 21 aboard Vancouver. Ward was back on the show grounds by the third week of showing and got to witness Dinan’s big win.

“The main difference is that I’ve been walking all the courses by myself. We go over them when I’m done, and other riders have been very helpful to me if I have questions about the courses, which is great,” Dinan said. “In the schooling area, I’ve been a little bit more independent because he can’t stand at the jump. I’ve had to do a little bit more of my own thinking in the schooling area.” Dinan has stepped up to the challenge, backed up by an excellent support staff at her Staysail Farm.

Dinan’s barn manager is Craig Pollard, and grooms Elizabeth Pedley and Camilla Petterson care for her grand prix horses. “I’ve learned how much people like Lee and Erica [McKeever] have to do with the success and how much depends on what they do. You realize how much of McLain Ward Inc. is Lee and Erica,” she said. “Even though it’s an individual sport once you’re in the ring, I think everyone who does well has a great team behind them.”

Dinan also has a phenomenal team of horses in her string. “I’m very lucky to have a barn full of great horses. Without them, I wouldn’t be anywhere,” she said. She bought Nougat du Vallet, or “Nugget”—“We kind of butchered the French!” Dinan said—in June 2011. The 11-year-old Selle Francais had had some grand prix mileage in France but wasn’t a big name. “He is a slightly small horse, and he’s fast but he’s also very rideable so you can turn quickly. You don’t need to set him up much in front of the fences—I can just set a pace and keep going,” Dinan said. “Even to the last jump in the jump-off, which was a tall vertical, I was going full speed, and he just set himself up. I don’t have to really half-halt much. He really tries to get out of the way of the fences. He’s very light off the ground, and he’s super careful. Sometimes if I try too hard, I interrupt his natural jump, so I’ve been learning to set it up and then let him do his thing. I think we’re figuring each other out.”

Dinan also has Vancouver, Amaroso, Super Trooper and the elder statesman Couletto K James to show in grand prix classes. She plans to ride one or two of them in the Olympic selection trials on March 21-24 in Wellington as a learning experience. Dinan bought Couletto K James, 16, three years ago as her first grand prix horse. The veteran has had mileage with Kim Prince, Katherine Miracle and Ward. “He’s been such a teacher for me. We try to give him an easy schedule because he’s no longer a spring chicken, but he’s still fantastic. He’s one of those horses that everyone at the horse show knows,” Dinan said.

Dinan graduated from high school in the spring of 2011 and then deferred her freshman year of college to devote a year to riding. She plans to start at Harvard University (Mass.) in August. “I’m excited to start school again because I like school,” she said. “I’m looking forward to being in the college environment and meeting new people. It will be hard because I’m having so much fun just riding. It will be a transition, but I’m looking forward to it.

“Everyone keeps asking me how much I’m going to ride while I’m in school, but I don’t really know. The answer to the question is, ‘Yes, I’ll ride.’ I just don’t know how much. I don’t want to shortchange the four years I have for college, but I also think I’d go into severe withdrawal if I stop riding. I want to find a happy medium.”

Dinan plans to major in either chemistry or biochemistry “I’m really interested in science, and they have a lot of great labs there. I don’t know what I want to do, but with a science background, I could go into either medical research or maybe be a veterinarian,” she continued.

With two top-three placings in the first three weeks of WEF, Dinan is proving that her grand prix education is already quite advanced.