Background: Prior studies comparing the mental healthcare utilisation (MHU) of Danish formerly deployed military personnel (FDP) with the general population have not included data on psychotherapy through the Defence or talking therapy with the general practitioner. This study included these and several other data sources in a comprehensive comparison of MHU between Danish FDP and civilians.Methods: First-time deployed military personnel (N = 10,971) who had returned from a mission to Kosovo, Afghanistan, Iraq or Lebanon between January 2005 and July 2017 were included. A sex and birth-year-matched civilian reference group was randomly drawn from the entire Danish non-deployed population (N = 253,714). Furthermore, a sub-cohort, including male FDP and civilians deemed eligible for military service, was defined. These cohorts were followed up in military medical records and registers covering the primary and secondary civilian health sectors from 2005 to 2018, and the rates of MHU were compared.Results: Approximately half of the initial help-seeking for FDP took place through the Defence (49.4%), and the remainder through the civilian healthcare system. When help-seeking through the Defence was not included, MHU was significantly lower among FDP in the main cohort during the first two years (IRR = 0.84, 95% CI: [0.77, 0.92]) compared to civilians. When help-seeking through the Defence was included, MHU was significantly higher among FDP compared to civilians both in the first two years of follow-up (IRR = 2.01, 95% CI: [1.89, 2.13]) and thereafter (IRR = 1.18, 95% CI: [1.13, 1.23]). In the sub-cohort, these differences were even more pronounced both in the first two years of follow-up and thereafter.Conclusions: MHU was higher among Danish FDP compared to civilians only when data from the Defence was included. The inclusion of data on both civilian and military healthcare services is necessary to evaluate the full impact of deployment on MHU among Danish FDP.