Exercise has an important role to play in the prevention of child and adolescent obesity. Recent school-based interventions have struggled to achieve meaningful and lasting changes to exercise levels. Theorists have suggested that this may, in part, be due to the failure to incorporate psychosocial mediators as they relate to behaviour change. Using a sample of 580 British schoolchildren, a model grounded in self-determination theory was explored to examine the effects of exercise goals on exercise motivation, leisure-time exercise behaviour and quality of life (QoL). Results of structural equation modelling revealed that adolescents perceiving themselves to be overweight and pressurized to lose weight, endorsed extrinsic weight-related goals for exercise. Extrinsic goals negatively predicted, whereas intrinsic goals positively predicted, self-determined motivation, which in turn positively predicted QoL and exercise behaviour. Furthermore, self-determined motivation partially mediated the effects of exercise goals on reported exercise behaviour and QoL. Multi-sample invariance testing revealed the proposed model to be largely invariant across gender. Results suggest that holding extrinsic exercise goals could compromise exercise participation levels and QoL. A role for teachers and parents is proposed with the aim of orienting young people towards intrinsic goals in an attempt to enhance future exercise behaviour and QoL.