Abstract: Adjunct or second-line therapies are increasingly being utilised among military populations to address treatment barriers and improve mental health outcomes. This study evaluated the psychosocial outcomes of an Australian peer-led adventure-based therapy program, Trojan’s Trek, for 60 ex-service military personnel (i.e., veterans, 56.7% male). Participants completed 1 of 6 Queensland Trojan’s Trek programs—a 6-day, live-in program consisting of 10–12 structured CBT-based group sessions and 11 nature-based activities—between March 2018 and March 2019. A non-controlled, within-subjects longitudinal design was utilised with assessment at pre-intervention, post-intervention, and 2-month follow-up. Changes in psychosocial outcomes were assessed using the Depression, Anxiety, and Stress Scale 21 (DASS-21), PTSD Checklist-5 (PCL-5), Positive and Negative Interactions Scale (PNI), General Self-Efficacy Scale (GSES), and the Life Satisfaction Questionnaire (LSQ). Mixed-factorial ANOVAs indicated a significant time x gender effect for anxiety (male F[2, 28] = 31.1, p < .001; female F[2, 19] = 4.05, p = .025); and stress (male F[2, 28] = 45.3, p < .001; female F[2, 19] = 9.98, p < .001). There were significant improvements between pre- and post-intervention for depression, anxiety (male only), stress, posttraumatic stress, perceived positive relationships with friends and family, perceived negative relationships with family and partners, self-efficacy, and life satisfaction. Improvements were maintained or established at 2-month follow-up for depression, anxiety (male and female), stress, posttraumatic stress, perceived negative relationships with friends, and self-efficacy. This study demonstrates utility of the Trojan’s Trek program for male and female Australian veterans with mental health difficulties. However, a controlled trial is required to determine program efficacy.