The current study examined the association between the possession of personal growth initiative (PGI) skills and symptoms of mental illness (i.e., depression, anxiety, and stress) and the potential mediating role of daily engagement of PGI skills during the COVID-19 pandemic. The participants consisted of 73 individuals (Mage = 41.62, SDage = 12.44) who were recruited from Amazon's Mechanical Turk (MTurk). Data for possessed PGI was collected in April 2020, and the daily survey started approximately 1 month later at the beginning of May 2020 for 29 days. Controlling for demographic variables, multilevel mediation models revealed significant association between PGI skills and PGI engagement. However, our hypothesized indirect effects were not found. When inspecting the direct effects controlling for the effect of PGI engagement and demographic variables, the paths between PGI skills and both depression and anxiety were negatively associated. When examining the effect of time, there were no significant changes during the period of 29 days in the levels of PGI skills among individuals. However, a significant fluctuation within individuals in how much participants have utilized PGI skills during the study period was found. The current study highlights the influence of PGI skills at both the between-individual as well as within-individual levels in predicting symptoms of mental illness, and it also uncovers the role of PGI skills by separating into general PGI skills and individual's daily engagement in utilizing the PGI skills in predicting symptoms of mental illness.