Not only does the 14-year-old from Corona ride and take care of the animals, she helps teach young children with special needs how to ride as well.
"I love seeing the smile on their faces," said Thornburg, "Because for me, having a disability of my own, the things that make me smile, it's fun seeing it makes them smile as well."
Thornburg was diagnosed with Type 1 diabetes when she was 15 months old, and she continuously wears an insulin pump. The straight-A student and volleyball player at Santiago High School also speaks to city leaders and schools about the symptoms and avoiding Type 1 misdiagnosis with the help of the non-profit group, Ease T1D.
Her focus at Trinity Riding Center is helping once-shy kids find their own confidence and independence, constantly cheering each on in their accomplishments while on horseback.
"You ask them, 'Do you want to trot?' and they're like 'Yeah! Yeah! Yeah!'" said Thornburg. "And you can tell at that point that they're just so overwhelmed with happiness."
"Sierra is an amazing young lady who has inspired us all," said Stephanie McGregor, founder of the Trinity Riding Center. "She has a huge heart for horses and for people."
"She is one cool kid. I'll tell you that," added Scott McGregor. "We're just really thankful to have Sierra as a part of our team."
After high school, Thornburg says she wants to be a forensic accountant, but says she'll never give up working with horses, calling moments like these inspiring.
"You think you're only helping others, when really you're also finding things out about yourself that you may have never known." said Thornburg.