Aim: It has been reported in the literature that patients with poor preoperative mental health are more likely to have worse functional outcomes following primary total hip and knee arthroplasty. We could find no studies investigating whether preoperative mental health also affects length of hospital stay following surgery. The aim of this study was to determine whether preoperative mental health affects length of hospital stay and long-term functional outcomes following primary total hip and knee arthroplasty. We also aimed to determine whether mental health scores improve after arthroplasty surgery and, finally, we looked specifically at a subgroup of patients with diagnosed mental illness to determine whether this affects length of hospital stay and functional outcomes after surgery. Method: Through a review of prospectively collected regional joint registry data, we compared preoperative mental health scores (SF-12 MH) with length of hospital stay and post-operative (1 and 5 years) functional outcome scores (Oxford and Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)) in 2279 primary total hip and knee arthroplasty surgeries performed in the Bay of Plenty District Health Board between 2006 and 2010. Results: Based on Pearson product–moment correlation coefficients, there was a significant correlation between preoperative mental health scores and post-operative Oxford scores at 1 year as well as post-operative WOMAC scores at both 1 and 5 years. There was no significant correlation between preoperative mental health and length of hospital stay. Mental health scores improved significantly after arthroplasty surgery. Those patients with a formally diagnosed mental illness had significantly worse preoperative mental health and function scores. Following surgery, they had longer hospital stays although their improvement in function was not significantly different to those without mental illness. Conclusion: The results of this study support reports in the literature that there is a correlation between preoperative mental health and long-term functional outcomes following primary total hip and knee arthroplasty. Patients with poor preoperative mental health are more likely to have worse functional outcomes at 1 and 5 years following surgery. No correlation between preoperative mental health and length of hospital stay was identified. Mental health scores improved significantly after surgery. Patients with mental illness had longer hospital stays and despite worse preoperative mental health and function had equal improvements in functional outcomes.