Abstract Background Drinking motivations within the UK military have not been studied despite the high prevalence of alcohol misuse in this group. Aims We aimed to characterize drinking motivations and their demographic, military and mental health associations in UK serving and ex-serving personnel. Methods Serving and ex-serving personnel reporting mental health, stress or emotional problems occurring in the last 3 years were selected from an existing cohort study. A semi-structured telephone interview survey examined participants’ mental health, help-seeking, alcohol use and drinking motivations. Results Exploratory factor analysis of drinking motivations in military personnel (n = 1279; response rate = 84.6%) yielded 2 factors, labelled ‘drinking to cope’ and ‘social pressure’. Higher drinking to cope motivations were associated with probable anxiety (rate ratio [RR] = 1.4; 95% confidence interval [CI] = 1.3–1.5), depression (RR = 1.3; 95% CI = 1.2–1.4) and post-traumatic stress disorder (RR = 1.4; 95% CI = 1.3–1.6). Higher social pressure motivations were associated with probable anxiety (odds ratio = 1.1; 95% CI = 1.0–1.1). Alcohol misuse and binge drinking were associated with reporting higher drinking to cope motivations, drinking at home and drinking alone. Conclusions Amongst military personnel with a stress, emotional or mental health problem, those who drink to cope with mental disorder symptoms or because of social pressure, in addition to those who drink at home or drink alone, are more likely to also drink excessively.