Sinclair made her debut for the senior team at age 16 at the 2000 Algarve Cup where she was the tournament's leading scorer with three goals. She scored seven goals for Canada at the 2002 CONCACAF Women's Gold Cup, which tied for the tournament's lead. Her three goals at the 2003 FIFA Women's World Cup helped lead Canada to the team's first fourth-place finish (a team best at the time since the inaugural 1991 FIFA Women's World Cup).