Horwill made his Super 14 debut against arch rivals the New South Wales Waratahs in the first game of the 2006 season, after not even being included in the Reds original squad.

[6][7][8] He made his competitive debut in Quins' season opener against Wasps FC on 16 October 2015 and immediately established himself as a first choice in the team.

[11] Horwill proved to be the find of the 2006 season, forcing his way into the starting line up and earning a place in the Wallabies spring tour, playing for Australia A in all three of the matches.

In 2007, Horwill made his debut for the Wallabies in a test match against Fiji playing the entire game, and was unlucky to not have been included in the 2007 Rugby World Cup squad.

[1] The following week, Australian coach Michael Cheika decided to leave Horwill out of his 31-man squad for the 2015 Rugby World Cup, a move which surprised some observers.