Looking for a serious challenge which combined horses and travel, Ms Donnelly said this race, which would push her to the limit was "absolute top of [her] bucket list".
She said she had followed the derby in previous years, but now, "I'm ready".
But on the home stretch she had to retire. Of the 41 riders who set out, 14 have retired before the finish line due to injury and general tiredness. Some had even been hospitalised but were now well enough to cheer from the sidelines.
The 20 men and 21 women from 13 different countries set off on the derby on August 4.
There was pre-race training from August 1 to August 3 and the final riders are expected to finish by August 14.
The nurse is one of two New Zealanders who were selected for the derby. Gareth Jones, 45, who is a New Zealander but lives in Ireland, is also competing.
Riders change horse every 40km and either live with herders or camp under the stars.
Krista Donnelly and a riding companion set off at 7am in misty conditions in Mongolia. Photo / Richard Dunwoody
Some of the riders are professional or semi-professional, others are enthusiastic amateur riders. They are all vying for the derby crown.
Competitors need to balance survival skills and horsemanship, they must endure the elements, semi-wild horses and unfamiliar food and terrain.
Anti-rainforest destruction charity Cool Earth is the race's official charity, but riders can also raise money for their chosen charity.
The entry fee for the 2016 Mongol Derby is close to $20,000, competitors are also required to raise more than $1000 for both Cool Earth and their chosen charity.
Additionally, the cost of her kit and other supplies for the race took the total cost to $30,000.
