As one of the frontline professionals during the coronavirus (COVID-19) pandemic, social workers can face tremendous pressure, which can lead to mental health problems. The objective of this study was to investigate whether a brief mindfulness-based intervention had short- and middle-term effects on social workers working during the COVID-19 pandemic. In a randomized controlled trial, 49 social workers were assigned to an experimental group (n = 28) or a waitlist control group (n = 21). The intervention was a four-week mindfulness-based program. The outcome variables were measured at pre- and post-test, and at one-month follow-up test. At post-test and follow-up, the workers in the experimental group reported higher mean scores in psychological flexibility and self-compassion and lower mean scores in depression compared to the workers in the waitlist control group. Although the experimental group also showed lower mean scores than the waitlist control group in anxiety and stress at both post-test and follow, the differences were not statistically significant. The findings suggest that a brief mindfulness-based intervention can improve psychological flexibility, self-compassion, and depression in social workers working during the COVID-19 crisis, leading to protected mental health, adding support to continuing development of such efficient mindfulness-based programs. Future studies should use larger samples and examine brief interventions’ working mechanisms.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12144-021-02150-3.