Pennie won a bronze medal at the Junior World Championships in 2000 and won the overall Junior World Cup Series in the same year. As an Under 23 athlete, she won a team bronze medal in 2004 and an individual bronze medal in 2005, both at the European Under 23 Championships. She competed in the individual slalom (K-1) event at the 2008 and 2016 Olympics and placed 17th and 6th, respectively.
She won four medals at the ICF Canoe Slalom World Championships with three silvers (K-1: 2006, 2014; K-1 team: 2015) and a bronze (K-1 team: 2007). She became European Champion in the K1 event in 2013. She won a total of 6 medals (2 golds, 1 silver and 3 bronzes) at the European Championships.