Her first disability sports event was at the age of 11, winning seven gold medals across several events. In 2008, at the age of 14, she was selected to represent Great Britain at the Summer Paralympic Games in Beijing, finishing 11th in the women's F42-46 javelin. She was the youngest member of the 2008 Great Britain team. The next year she won her first significant medals when she competed in the 2009 IWAS World Junior Championships in Switzerland, winning silver in the F46 discus and bronze in the F46 javelin. 2009 was the last year Arnold competed at championships in discus, concentrating fully on the javelin. The following year she improved on her javelin medal when she took silver at the 2010 IWAS World Junior Championship and then took gold at the 2011 Championship which was held in Dubai. The year 2011 saw Arnold step up to the senior level representing Britain at the IPC Athletics World Championships in New Zealand; she finished third in the F46 javelin, taking the bronze medal. In 2012, she took the silver medal at the IPC Championships. Her personal best at F46 javelin was recorded at 35.88m in June 2012 at the Welsh Championships in Cardiff, ranking her as the world number two in her event going into the 2012 Paralympics. Arnold's personal best was increased at the 2012 Summer Paralympics in London with a throw of 36.27m. On 22 July 2013, Arnold competed in the IPC Athletics World Championships in Lyon. She won gold with a winning throw of 37.45m, furthering her personal best.