Social support is a key determinant of mental health across multiple populations and contexts. Little is known about social support among UK (ex-)military personnel, especially those with combat injuries following deployment to Afghanistan. This study aimed to investigate the level of perceived social support and its associations with mental health among injured and uninjured UK (ex-)military personnel. An analysis of baseline data from the Armed Services Trauma Rehabilitation Outcome (ADVANCE) prospective cohort study was performed. A representative sample of male UK combat-injured personnel was compared with a frequency-matched sample of uninjured personnel. Validated questionnaires were completed including the Multidimensional Scale of Perceived Social Support (MSPSS). MSPSS score was transformed using linear splines with a knot at ≥ 55. Multivariable logistic regression analyses examined associations between perceived social support and mental health. In total, 521 combat-injured participants (137 with amputations) and 515 uninjured participants were included. Median MSPSS score was 65 (interquartile range [IQR] 54–74). Injured and uninjured participants reported similar MSPSS scores, as did those injured with amputations, and non-amputation injured participants. For each one unit increase in MSPSS score (for scores ≥55), the odds of post-traumatic stress disorder decreased (adjusted odds ratio [AOR] 0.93, 95% confidence interval [CI] 0.91 to 0.96). No such association was found with MSPSS scores below 55 (AOR 0.99, 95% CI 0.97 to 1.01). Similar results were observed for depression and anxiety. Perceived social support may be a target for intervention within this population, irrespective of injury status.