Physical activity (PA) has been suggested as a strategy to promote youth mental well-being, yet most adolescent girls are insufficiently active. While PA has been identified as a strategy for promoting positive body image, the way body image and mental health are experienced concurrently, and in the context of PA, has not been explored. The purpose of this study was to explore adolescent girls’ mental health and body image to better understand how PA experiences are navigated and negotiated. Semi-structured interviews were conducted with seven adolescent girls (ages 15-17 years). Following a qualitative description approach, data were analyzed using reflexive thematic analysis. Based on the findings, four themes were identified: (1) body image is impossible to separate from mental health, (2) type and context of PA can be both protective and threatening, (3) social comparisons are unavoidable, and (4) peers are essential for navigating body image and mental health. Overall, organized sport was described as isolating and discouraging with potential poor effects on body image and mental health. Girls recognized PA as essential to well-being; however, they described PA as difficult to maintain when experiencing negative body-related self-thoughts. Given these qualitative results, strategies to promote PA adherence while minimizing negative experiences may require a multifaceted approach, prioritizing social and emotional well-being, management of body image concerns, and promotion of self-confidence. These findings help advance the conceptualizations of body image and mental health and further the understanding of the interplay between these constructs in the context of adolescent girls’ PA.