Cassie Patten attended Plymouth College on a swimming scholarship from the age of 11. She had been a medalist at the distance freestyle events at the UK National Championships before she made the decision to switch to the long distance events but it was in the open water events that her talent became evident. At the 2007 World Championships in Australia she finished runner-up by only a second to champion Lara Ilchenko in the 10 km open water race. She was chosen for the Beijing Games for two events, the 800 freestyle in the pool and the open water event, and reached the final of the 800 though she would place last in the final. Patten was in a breakaway pack of leaders in the open water swim and held onto third position at the finish behind Ilchenko and team-mate Keri-Anne Payne. Post Beijing she began to experience problems with her shoulder which, after an abortive attempt to qualify for the 2012 Games, forced her to retire in October 2011 at only 24 years of age.