Born in Scotland, McCallum emigrated to Australia with her family at the age of four.[2][3] She played for Pali Blues in the American USL W-League in 2008.[4] Following her debut game for the Blues, she was selected for the W-League team of the week as a result of scoring two goals against Ventura County Fusion. Pali Blues went on to win the W-League championship in that season.
McCallum is arguably one of the best produced at her age group and was considered one of the best young talents in the world.
Such is her legacy at youth level that in 2019 Professional Footballers Australia named the Australian Young Player of the Year award the Collette McCallum Medal in her honour.
The 132nd capped Matilda played every Australian fixture at the 2007 FIFA Women’s World Cup (China 2007) & 2011 FIFA Women’s World Cup (Germany 2011) helping the team reach the quarter finals in each tournament.