To evaluate the effect of Veterans RAND 12-item health survey mental composite score (VR-12 MCS) on postoperative patient-reported outcome measures (PROMs) after undergoing lateral lumbar interbody fusion. Retrospective data from a single-surgeon database created 2 cohorts: patients with VR-12 MCS ≥ 50 or VR-12 MCS < 50. Preoperative, 6-week, and final follow-up (FF)- PROMs including VR-12 MCS/physical composite score (PCS), 12-item Short Form health survey (SF-12) MCS/PCS, Patient-Reported Outcomes Measurement Information System Physical Function (PROMIS-PF), Patient Health Questionnaire-9 (PHQ-9), visual analogue scale (VAS)-back/leg pain (VAS-BP/LP), and Oswestry Disability Index (ODI) were collected. ∆6-week and ∆FF-PROMs were calculated. Minimal clinically important difference (MCID) achievement rates were determined from established cutoffs from the literature. For intercohort comparison, chi-square analysis was used for categorical variables, and Student t-test for continuous variables. Seventy-nine patients were included; 25 were in VR-12 MCS < 50. Mean postoperative follow-up time was 17.12 ± 8.43 months. The VR-12 MCS < 50 cohort had worse VR-12 PCS, SF-12 MCS, PROMIS-PF, PHQ-9, VAS-BP, and ODI scores preoperatively (p ≤ 0.014, all), worse VR-12 MCS/PCS, SF-12 MCS, PROMIS-PF, PHQ-9, and ODI scores at 6-week postoperatively (p ≤ 0.039, all), and worse VR-12 MCS, SF-12 MCS, PROMIS-PF, PHQ-9, VAS-BP, VAS-LP, and ODI scores at FF (p ≤ 0.046, all). The VR-12 MCS < 50 cohort showed greater improvement in VR-12 MCS and SF-12 MCS scores at 6 weeks and FF (p ≤ 0.005, all). The VR-12 MCS < 50 cohort experienced greater MCID achievement for VR-12 MCS, SF-12 MCS, and PHQ-9 (p ≤ 0.006, all). VR-12 MCS < 50 yielded worse mental health, physical function, pain and disability postoperatively, yet reported greater improvements in magnitude and MCID achievement for mental health.