Objective <p>This study aims to evaluate the long-term impact of the six-week online mindfulness intervention course “Mindfulness Living With Challenge (MLWC)” on the mental health of undergraduate nursing students in Beijing, China, through a three-month follow-up.</p> Methods <p>A three-month follow-up was conducted upon completion of the six-week MLWC course. Depression-Anxiety-Stress Scale (DASS-21), Chinese Short Formed Five Facets of Mindfulness Questionnaire (FFMQ-SF), and the Perceived Social Support Scale (PSSS) were used to evaluate participants’ mental health, mindfulness, and perceived social support. Generalized estimation equations (GEE) were used to evaluate the effects of intervention and time. A causal mediation model was constructed to explore the mediation effects of perceived social support between mindfulness and mental health.</p> Results <p>Among 108 participants (52 intervention, 56 control), MLWC significantly reduced stress and anxiety levels, and improved mindfulness and social support levels among undergraduate nursing students (<i>P</i> &lt; 0.05), but long-term effects were not obvious in improving participants’ mental health. Perceived social support mediated the relationship between the mindfulness intervention and mental symptoms (<i>P</i> &lt; 0.05).</p> Conclusion <p>Online mindfulness interventions may temporarily alleviate mental health challenges in nursing students, with social support playing a mediating role. Sustained benefits may require regular practice, highlighting the need for ongoing mindfulness integration into training programs.</p> Trial registration <p>Chinese Clinical Trial Registry (ChiCTR), ChiCTR2200058103; Registered on March 29, 2022, <a href="http://www.chictr.org.cn/showproj.aspx?proj=136413">http://www.chictr.org.cn/showproj.aspx?proj=136413</a>.</p>