Physical factors such as frequency of low back pain, sensory abnormalities in the lower extremities, smoking history before surgery, and preoperative mental health status as predictors of operative outcomes have been growing as areas of interest in the field of degenerative lumbar spinal stenosis (DLSS). This study aimed to investigate the correlation between the preoperative Short Form-36 Mental Component Score (SF-36 MCS) and long-term prognosis after decompression surgery for DLSS. In total, 198 patients were enrolled in this study. The Oswestry Disability Index (ODI) and Rolland Morris Disability Questionnaire (RMDQ) were used to evaluate spinal functional outcomes. The SF-36 questionnaire was used and analyzed by classifying it into physical component score (PCS) and mental component score (MCS). The SF-36 MCS was divided into role limitations caused by emotional problems, social functioning, vitality, and emotional well-being. In the correlation between preoperative MCS and ODI improvement, the r value was -0.595 (P < .05) at 12 months postoperatively. ODI improvement at 12 months after decompression surgery showed a statistically significant and strong negative correlation with preoperative MCS. In the correlation between preoperative MCS and RMDQ improvement, the r value was -0.544 (P < .05) at 12 months postoperatively. Therefore, RMDQ improvement 12 months after decompression surgery showed a strong negative correlation with preoperative MCS. Regarding the correlation between preoperative MCS and SF-36 PCS improvement, the r values were 0.321 (P < .05) at 6 months postoperatively and 0.343 (P < .05) at 12 months postoperatively. Therefore, SF-36 PCS improvement at 6 and 12 months after decompression surgery showed a strong positive correlation with preoperative SF-36 MCS scores. Preoperative SF-36 MCS is a factor that can predict the prognosis of patients who underwent decompression surgery for lumbar spinal stenosis for at least 1 year postoperatively.