Born in Dundee, Scotland, then raised in England, Ellie moved to Canada to work as a ski tour operator, after graduating from university. Having completed her first race aged 22, running the Great North Run in a time of 1 hour 59 minutes and 57 seconds, Ellie initially started running for fun.
After experimenting with longer distances, she began her ultramarathon career in 2008, taking on the 30-mile Knee Knackering North Shore Trail Run, known for its steep ascent and descent. At her first attempt at 100K, Greenwood set a course record at the Elk-Beaver 100K on Vancouver Island in 2010, qualifying to represent Team GB at the 100K World Championships. Ellie went on to take gold and lead Britain to first place in the team event. In the same year she set course records in the 125K Canadian Death Race and the Knee Knackering North Shore Trail Run.