Objective: Poor body image is a public health issue. Mothers are a key influence on adolescent girls’ body image. This study evaluated an accessible, scalable, low-intensity internet-based intervention delivered to mothers (Dove Self Esteem Project Website for Parents) on mothers’ and their adolescent daughters’ body image and psychosocial well-being. Method: British mother-daughter dyads (N = 235) participated in a cluster randomized controlled trial (assessment-only control; mothers viewed the website without structured guidance [website-unstructured]; mothers viewed the website via a tailored pathway [website-tailored]). Dyads completed standardized self-report measures of body image, related risk factors, and psychosocial outcomes at baseline, 2 weeks post-exposure, 6-week, and 12-month follow-up. Results: Dyadic models showed that relative to the control, mothers who viewed the website reported significantly higher self-esteem at post-exposure (website-tailored), higher weight esteem at 6-week follow-up (website-tailored), lower negative affect at 12-month follow-up (website-tailored), engaged in more self-reported conversations with their daughters about body image at post-exposure and 6-week follow-up, and were 3–4.66 times more likely to report seeking additional support for body image issues at post-exposure (website-tailored), 6-week, and 12-month (website-tailored) follow-up. Daughters whose mothers viewed the website had higher self-esteem and reduced negative affect at 6-week follow-up. There were no differences on daughters’ body image, and risk factors among mothers or daughters, at post-exposure or follow-up. Tailoring website content appeared beneficial. Conclusions: This intervention offers a promising ‘first-step’ toward improving psychosocial well-being among mothers and daughters. In order to further optimize the intervention, future research to improve body image-related outcomes and to understand mechanisms for change would be beneficial.