First responders and other public safety personnel (PSP) experience high rates of mental health problems and face barriers to accessing mental healthcare. Internet-delivered cognitive behavioural therapy (ICBT) is an effective and accessible treatment for various mental health concerns. Canadian PSP report favorable attitudes toward ICBT, and preliminary outcomes demonstrate that they benefit from it. Expanding on this research, the current study consisted of a longitudinal observational study of 560 Canadian PSP who participated in ICBT. It was designed to assess the longer term effectiveness of ICBT and moderators of outcomes by gender, linguistic and occupational group, and years of occupational experience. We evaluated symptom change at 8, 26, and 52 weeks post-enrollment, and results among PSP who had elevated clinical scores, showed large reductions (Hedges’ g) in symptoms of depression (g = 1.3), anxiety (g =1.48), posttraumatic stress (g =1.24), panic (g =1.19), and anger (g =1.07) and moderate reductions in symptoms of social anxiety (g =.48–.56). Moderator analyses revealed modest differences in pre-treatment symptoms among certain groups but no group differences in symptom change over time. Clients showed good completion of treatment materials and reported high treatment satisfaction. The results suggest further study of ICBT tailored to PSP is warranted, including evaluating ICBT tailored for PSP in other countries.