Mantell made his international senior debut for the national squad on 9 November 2005 versus Ireland at Beeston. He was a member of the England squad that finished fifth at the 2006 Men's Hockey World Cup. He competed at the 2010 and 2011 Champions Trophy, and was also part of the winning squad at the 2009 EuroHockey Championship. Mantell has also represented Great Britain and was part of the side that returned victorious from the Olympic qualifying event in Chile, as well as competing at the 2008 Summer Olympics. He also competed at the 2006, 2010 and 2014 Commonwealth Games.
Mantell was born in Bridgwater, Somerset. He was educated at Millfield School in Street, followed by the University of Birmingham. Mantell has played club hockey for Reading and Bristol Firebrands, as well as for HGC in the Netherlands and Mumbai Magicians in the Hockey India League.