Workplace supportive supervisor interventions offer an effective, though underutilized mechanism to bolster employee well-being, which may have important benefits particularly for understudied groups such as military veterans in the civilian workforce. The present study employed a two-wave daily diary study to test the effectiveness of a supportive supervisor training on positive and negative emotions of veteran employees. Daily diaries are instrumental to understanding well-being, in that they accurately capture emotions as they are experienced without retrospective biases that reflect more global emotional assessments. Each wave (baseline and 6 months later) comprised emotion reports over 32-day periods. Thirty-five organizations were randomized into intervention and control groups; 144 veterans (91% men) participated in the daily diaries at baseline. The training significantly improved well-being facets including improvements in unactivated positive (i.e., calm) emotions at follow-up. Two significant moderation effects were also revealed for PTSD screening. For veteran employees with positive PTSD screens, the intervention functioned to reduce negative emotions. For those employees with negative PTSD screens, the intervention enhanced positive emotions. Our work highlights the benefit of workplace supervisor support to positive and negative employee mood. Practitioner points: Our supervisor support training represents an evidence-based programme that improves employee positive emotions. Positive emotions were increased among veterans who did not screen positive for PTSD (i.e., most veterans). Veterans with positive PTSD screens may benefit from enhanced supervisor support resulting in a reduction of negative emotions, providing supportive evidence for supervisor mental health awareness training.