Background Few studies have investigated wellbeing and burden in carers of people with severe multiple sclerosis (PwSMS). Objectives To assess the impact of providing care to PwSMS, and explore variables associated with perceived carer burden. Methods Cross-sectional assessment of health-related quality of life (HRQOL), mood symptoms (Hospital Anxiety and Depression Scale, HADS), and perceived carer burden (22-item Zarit Burden Interview, ZBI) in 78 PwSMS carers. Multivariate linear regression explored carer and PwSMS factors associated with ZBI score. Results Carers (61% women, mean age 60.2years, 53% spouse/partner) had significantly lower HRQOL (all SF-36 scales) than the norm, especially for Role Limitation Emotional/Physical, and Emotional Wellbeing. Sixty-eight percent had pathologic (≥8) Anxiety, and 44% had pathologic Depression scores on HADS. Nonetheless, perceived carer burden was only moderate (mean ZBI score 35.6, SD 14.3). High carer anxiety (p < 0.0001), low household income (p = 0.009), and living with the PwSMS (p = 0.02) were independent predictors of perceived burden. Conclusions Caring for PwSMS has a detrimental effect on HRQOL and psychological wellbeing. High carer anxiety, low economic status, and living in predict higher burden. It is crucial to recognize PwSMS carers as full partners in the provision of care, and to respond to their own needs.