Patients treated with immune checkpoint inhibitors (ICIs) often experience unique immune-related adverse events (irAEs), and the previous studies demonstrated an association between irAEs and better outcomes in patients with ICI treatment for advanced non-small cell lung cancer (NSCLC). However, the correlation between the occurrence of mild and severe irAEs and prognosis remains unclear. Additionally, little is known regarding the association between the timing of mild and severe irAEs and clinical outcomes. We retrospectively conducted a multicenter study of advanced NSCLC patients treated with ICI monotherapy. Of the 222 patients, 79 patients (35.6%) experienced at least one irAE, and most were of grade 1 or 2 (mild) (26.6%). The most common irAEs were pneumonitis (n = 21, 9.5%) and skin-related adverse reactions (n = 19, 8.6%). The median progression-free survival of all patients treated with ICIs was 3.2 months. Patients experiencing irAEs had a better prognosis than those without such events (6.5 vs. 2.6 months, p = 0.004), and mild irAEs were associated with the best prognosis. The difference in overall survival between mild and severe irAEs was significant (34.3 vs. 17.3 months, p = 0.021). We further analyzed differences between patients with irAEs occurring at 3 or 6 weeks, and found that the earlier the occurrence of mild irAEs, the better the prognosis; however, the opposite was true for severe irAEs. In summary, patients with early occurring mild irAEs showed better clinical outcomes, whereas those with early severe irAEs tended to show poorer clinical outcomes.© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.