Cullen's father was a golfer and her mother played professional squash. She first played hockey aged nine, when living in Kenya until she was twelve. Her parents still live in Kenya where they run the Hemingways hotel near Watamu. She boarded at the independent Oakham School in Rutland. She then studied for a BA in Business Studies (Strategy & Entrepreneurship) at the Nottingham Business School, gaining a 2.1 in 2006. She works as a Business Development Consultant for The Kennel Club. Her brother Gray Cullen played junior rugby for England and in 2009 debuted for the Kenya national rugby union team.
Cullen made her senior international debut for the England women's national field hockey team in 2003. Also, she was named in the International Hockey federation World Star XI following the World Cup, as well as 2006 Great Britain Hockey Athlete of the Year and Hockey Writers' Club Player of the Year. She participated in the team which won Bronze at the 2006 Commonwealth Games in Melbourne. She is currently playing for Leicester Hockey Club, which is based at the Stoneygate Preparatory School in Great Glen.