This study aimed to assess the efficacy of a guided online mindfulness-based intervention (iMBI) for community residents experiencing emotional distress during the COVID-19 pandemic in Hong Kong. In a randomized controlled trial, 64 participants were recruited from collaborating community-based mental health service units in Hong Kong and assigned to either the treatment (n = 32) or control (n = 32) groups. The treatment group received a guided iMBI consisting of 16 online modules, weekly telephone counseling, and two half-day online workshops on mindfulness practice. In contrast, the waitlist control group did not receive any intervention during the initial stage. Using a 2 (two groups) × time (pre versus post) repeated measures linear mixed model and one-way analysis of variance, authors demonstrated that the treatment group experienced a significantly larger reduction in anxiety and depressive symptoms with a large effect size compared with the control group. Additionally, the treatment group showed a significantly greater improvement in mindfulness with a moderate effect size. The findings support the effectiveness of guided iMBI for community residents experiencing emotional distress during the COVID-19 pandemic in Hong Kong.