He first played for Edinburgh Rugby and then Bristol Rugby as well as representing the Scotland national team His position of choice is Flanker. He was called up to the Scotland squad for the 2008 Six Nations Championship.
He was also selected in the Scotland squad for the 2011 Rugby World Cup in New Zealand.
He started his professional career at Edinburgh Rugby and, after falling out of favour under Head Coach Alan Solomons, was sent out on loan to Bristol Rugby for the second half of the 2013-2014 season, making the deal permanent before the 2014-2015 season after a string of impressive performances in the number seven jersey. On 14 January 2015, Rennie announced his retirement from rugby with immediate effect due to a neurological problem.