BACKGROUND: The purpose of this study is to determine the impact of total knee arthroplasty (TKA) on mental health. METHODS: A total of 205 patients who underwent primary TKA with baseline and 1-year postoperative Short Form-12 Mental Component Score (MCS) were included in this retrospective analysis. Eighty-five (41%) patients had a preoperative MCS less than 50 points, while 120 (59%) patients had a preoperative MCS over 50 points. Two groups were assigned to the patients based on their preoperative MCS: low MCS <50 and high MCS >50. RESULTS: A preoperative MCS less than 50 points was predictive of greater improvement in MCS at 1 year after TKA (P < .001). Patients with low MCS improved by a mean of 10.6 points from 39.1 +/- 8.6 points preoperatively to mean of 49.7 +/- 10.7 points 1 year after TKA (P < .001). Patients with a high MCS decreased by a mean of 3.5 points from 60.01 +/- 6.0 points preoperatively to mean of 56.6 +/- 6.8 points 1 year after TKA (P < .001). This remained higher than the postoperative MCS of the patients with a low MCS, 49.7 +/- 10.7 (P < .001). The patients with a high MCS had greater improvement in the Short Form-12-Physical domain (14.8 points) than the patients with a low MCS (9.2 points, P < .001). CONCLUSION: Patients with lower baseline mental health had greater improvement in postoperative mental health following TKA than patients with higher baseline mental health. Low preoperative MCS was associated with less improvement in patient-reported outcome measures. Patients with lower baseline mental health scores before TKA benefit mentally and physically from the procedure.