Background: Increasing health awareness in health promotion is considered as one of the less stigmatized interventions for improving help-seeking behaviors and total well-being. This study aimed to explore the short-term and long-term effectiveness of the health-awareness-strengthening lifestyle (HASL) program on Taiwanese young adults with at-risk mental state. Methods: A pre- and post-test randomized trial was conducted on 92 young adults with at-risk mental state. The HASL program was provided to the experimental group as intervention, and it was only provided to the control group passively by request after the post-test for ethical reasons. The program was conducted once every six weeks, 60–90 min per session, for a total of three times. Mental health risk, anxiety level, health promotion lifestyles, quality of life, physiological index, and physical exercises were assessed one week before and after the program for both groups and followed up to 6 and 12 months for experimental group only. Results: Compared to the control group, those in the experimental group showed significant improvements regarding anxiety level, health promotion lifestyles, and quality of life one week after participating in the program. Furthermore, the experimental group also showed an additional long-term positive effect on mental risk, physical exercises, and physical health after the follow-ups. Conclusions: The outcomes highlighted the interventions of the HASL program leading to more positive health effects on young adults with at-risk mental state. The implementation of similar clinical service is recommended for young adults with at-risk mental state.