Background <p>Family carers (caregivers) of cancer patients often have poor mental health, which may adversely affect patient wellbeing and health service use. We examined the evidence for this using a systematic review and meta-analyses. </p> Methods <p>We systematically searched online databases for studies reporting associations between carer psychological health and either: (a) psychological health of patients with cancer they cared for; or (b) health service use in carers or patients. Studies’ risk of bias was assessed using the Joanna Briggs Institute Critical Appraisal Tool. We undertook meta-analyses to estimate pooled correlations for the most commonly reported associations between carer and patient psychological health. For associations between carer psychological health and carer and/or patient health service use, we conducted a narrative synthesis.</p> Results <p>Our search identified 11,911 records, 169 of which were eligible. The majority of studies were of high-to-moderate quality. Carers who were distressed, depressed, anxious, or had poor scores in the mental component of quality of life were significantly more likely to be caring for patients with these same outcomes (pooled effect sizes ranged from 0.28 to 0.42; all <i>p</i> &lt; 0.001). Subgroup analyses by gender, disease stage, and study quality revealed no substantial differences. Carers with poor psychological health used general practice, mental healthcare, and hospital services more frequently than those who were psychologically healthy, and the patients they cared for used more medications and had more frequent emergency presentations.</p> Conclusion <p>The inclusion of carers alongside patients in early psychosocial care may improve family outcomes and reduce health service use.</p>