McWilliams was born in Newcastle, New South Wales and played his junior rugby league for the Singleton Greyhounds.
[2][3] In 1995, while attending Singleton High School, he was selected in the Australian Schoolboys squad that toured Great Britain and France.
[4] McWilliams was a Newcastle Knights junior before joining the Hunter Mariners, where he represented the Australian Super League under-19 side in 1997.
[9] In 2005, McWilliams played just seven NRL games for the Cowboys, spending the majority of the season with the North Queensland Young Guns.
On 17 September 2005, he started at lock in the Young Guns' 36–6 Queensland Cup Grand Final win over the Burleigh Bears.