At the age of 13, Ellie was the youngest British athlete at the 2008 Summer Paralympics in Beijing, competing in the 50m, 100m and 400m freestyle, 50m butterfly, and 200m Individual Medley. She won gold medals in the 100m and 400m freestyle events.
On 1 September 2012, Simmonds repeated her gold performance to win the 400m freestyle at the 2012 Summer Paralympics in London, in which she took five seconds off the World Record time. Two days later, on the evening of 3 September, she took Gold in the 200m Individual Medley, breaking the World Record that she had set in the qualifying round that morning.
On 12 September 2016, at the Rio Paralympics, Ellie defended her Gold medal for the 200m individual medley setting a new world record, the first below 3 minutes at 2:59.81 Simmonds also won a bronze medal in the 400m freestyle at the 2016 paralympics.
In addition, Ellie has won ten gold World Championship titles.