While there is increasing recognition of the need to go beyond measures of mental ill health, there is a relative dearth of validated tools for assessing mental well-being among adolescents. The Warwick-Edinburgh Mental Well-being Scale (WEMWBS) is a promising tool for use in this context, and this study evaluated its use in an Australian context. The WEMWBS was completed by 829 Western Australian adolescents, aged from 13 to 16 years old, drawn from Grades 8, 9 and 10 in seven separate high schools. Using confirmatory factor analytic techniques, the utility of the full 14-item scale was not supported, but good fit for a previously validated seven-item short version (SWEMWBS) was supported. Strong measurement invariance was demonstrated across age, and weak measurement invariance was demonstrated across gender. The scale has good internal reliability. There were no differences in SWEMWBS scores across Grades 8 to 10. Overall, the SWEMWBS represents a useful tool for educational, developmental, and school psychologists investigating positive mental wellbeing in younger adolescents.