Mendoza was just 10 when she suffered the most serious injury of her career.
Now 19, she looks back on that day and the recovery period as the moment she recognized her hunger to become the best.
"I fell off and the horse trod on my back and my ribs and then my lung," Mendoza told CNN.
"I was off for quite a while -- several months actually -- and I still really wanted to go back riding.
"I went back before I was actually allowed and it showed how much I really wanted to do it."
Her fearless nature is plain to see when she competes against some of the biggest names in the world of equestrian.
Named Longines Rising Star at last year's FEI Awards, Mendoza enjoyed a stellar 2015.
She won 10 events and made history by appearing for Great Britain at the European Championships -- making her the youngest to represent the team for nearly 40 years.
Her efforts at the championships held in Aachen, Germany helped Britain to finish fourth, ensuring qualification for the Olympic Games in Rio.
Her success has coincided with a decision to move to Eindhoven, Netherlands, where she has spent the past two years training.
"I think the move, as a rider, has changed me a bit," she said.
"I've had more training put into me, more flat work and it has really developed my skills as well as my passion.
"Traveling from England to Europe is eight hours every time and it's a lot for the horses to do and it's the best decision we've made. We love it out there."
The next 12 months could be the teenager's most exciting yet with the chance of appearing at the Olympics a real possibility.
After helping Britain clinch qualification, she is hopeful that a place on the team won't elude her -- though she is refusing to get carried away.
"It is my dream to be selected for Rio," she said. "I think to do that at 19 would be unbelievable, so I'm hoping.
"I need to get better results than the other team members because they're so much more experienced.
"I just need to prove myself and keep being consistent."