This study tests a model derived from Self-Determination Theory (SDT) (Deci and Ryan, 2000) to explain the mechanisms by which non-modifiable factors influence dietary self-care in adolescents with type 1 diabetes (n = 289). SEM analyses adjusted for HbA1c levels revealed that longer diabetes duration and female gender were indicative of poorer dietary self-care. This effect was mediated by contextual and motivational factors as posited by SDT. Poorer autonomy support from practitioners was predominant in girls with longer diabetes duration. Perceived autonomous motivation and self-efficacy were indicative of greater autonomy support, and led to better dietary self-care.