Imogen Allen, 155, of Pot Kiln Lane in Woodcote, won the Mercers Challenge Cup in the showjumping classes.
Her event included 1m jumps and was open to anyone living within 15 miles of Henley town hall.
She competed on her 11-year-old Irish Connemara pony Crindle Miss Sunshine.
All the other entrants were on horses and most were adults.
Imogen, who attends Cranford House School, won £15 and a sash. She was watched by her parents Tessa and Chris and sister Mia, 12.