Details of Mac's military service are sketchy as he has rarely discussed his past to his colleagues but it has been implied that Mac had a distinguished and decorated career as a Marine. There are conflicting details about the rank at which he was discharged as his DD Form 214, as shown in the Season 2 finale, states that he was a Sergeant. In the same episode he stated that he was Lieutenant while serving in Beirut and in Season 6, in a flashback, he is shown to be a Major.