Background: Although caregivers of patients with eating disorders usually experience a heavy caregiving burden, the effects of social support on caregivers of patients with eating disorders are unknown. This study aimed to investigate how social support for mothers who are caregivers of patients with an eating disorder improves the mothers’ mental status and, consequently, the symptoms and status of the patients. Methods: Fifty-seven pairs of participants were recruited from four family self-help groups and one university hospital in Japan. Recruitment was conducted from July 2017 to August 2018. Mothers were evaluated for social support using the Japanese version of the Social Provisions Scale-10 item (SPS-10), self-efficacy using the General Self-Efficacy Scale, loneliness using the University of California, Los Angeles Loneliness Scale, listening attitude using the Active Listening Attitude Scale, family functioning using the Family Assessment Device, depression symptoms using the Beck Depression Inventory (Second Edition), and psychological distress using the Kessler Psychological Distress Scale. Patients were evaluated for self-esteem using the Rosenberg Self-Esteem Scale, assertion using the Youth Assertion Scale, and their symptoms using the Eating Disorder Inventory. We divided the mothers and patients into two groups based on the mean score of the SPS-10 of mothers and compared the status of mothers and patients between the high- and low-scoring groups. Results: High social support for mothers of patients with eating disorders was significantly associated with lower scores for loneliness and depression of these mothers. We found no significant differences in any patient scores based on mothers’ level of social support. Conclusions: For patients with eating disorders, social support for a caregiver cannot be expected to improve their symptoms, but it may help prevent caregiver depression and loneliness.