Informal carers play a vital role in supporting Australians living with a mental illness, including during the acute phases of illness; however, little is known about their impact on length of hospital stay. We aimed to investigate the impact of having a carer and of carer burden on length of hospital stay for mental health. Two Australian datasets were used. Data from the 2010 National Survey of High Impact Psychosis (n = 1825) were used to investigate the impact of having versus not having a carer on length of hospital stay for mental health. Data from the UQ Carer Survey 2016 (n = 105), a convenience sample of mental health carers, were used to investigate the impact of weekly hours of care (a measure of objective carer burden) on length of stay. Multiple logistic regression and correlation analyses were performed to investigate the association between carer status/burden and length of stay. Having a carer was associated with a significantly longer length of hospital stay; however, this relationship was no longer significant after adjusting for diagnosis, global functioning, depressive symptoms, deliberate self-harm, mental health outpatient contacts and type of admission. Weekly hours of care did not significantly impact on length of stay. Patients with carers had poorer functioning which may be related to longer stays. Our analysis was not able to look at subgroups of carers with different needs. Future work is required to determine other components of the admission and discharge process where having a carer is influential.