Objective: This study, rooted in narrative psychology, examined the effectiveness of the proactive self-narrative diary intervention for adults’ mental health during the COVID-19 pandemic. Methods: Four hundred twenty-four people aged 18 to 45 were recruited and divided into three groups: ‘narrative’, ‘descriptive’, and ‘control’. On three consecutive days, participants in the ‘narrative’ group were asked to narrate their daily experiences related to the pandemic. At the same time, people from the ‘descriptive’ group were asked to answer some questions related to their daily activities, and the third group did nothing. The effect of the intervention on mental health was assessed in three follow-up measurements (a day, two weeks, and two months after the end of the intervention). Self-narrative inclination, awareness of narrative identity, and reflexivity were analyzed as factors modifying the effect of the self-diary intervention. Results: The results indicate a significant decrease in mental health problems over time, but there were no differences between the three groups. Contrary to expectations, none of the abovementioned factors was a moderator of the effectiveness of the intervention. Conclusions. This experiment with three follow-ups did not support the self-diary intervention outcome of a significant decrease in mental health problems and an increase in positive mental health. The findings are discussed, referring to the theoretical framework and the results of other studies verifying the effectiveness of various narrative interventions on mental health during the pandemic.