Background: Online gambling and sports betting have recently been legalized and expanded in North America. We aimed to evaluate long-term changes in the frequency of gambling-related helpline contacts in the province of Ontario following the launch of a government-operated online gambling platform (PlayOLG) in January 2015 and the expansion of private online gambling, including single-event sports betting, in April 2022. Methods: We conducted an interrupted time-series analysis of contacts with Ontario’s free 24-hour mental health and addictions helpline (ConnexOntario) between January 2012 and September 2025. We used segmented linear regression to compare immediate, gradual, and overall changes in monthly rates of gambling-related contacts after gambling expansion in January 2015 and April 2022 overall and by age and gender subgroups. Results: Over 13 years, ConnexOntario received 745 716 contacts, including 37 087 (5.0%, mean age 39 yr, 68.2% men) related to gambling. The mean monthly rate of gambling-related contacts per 1 000 000 individuals aged 15 years and older was stable over time before the launch of PlayOLG (13.4); the rate increased after Play-OLG’s launch (17.0) and further increased following privatization (26.2). Compared with the scenario in which PlayOLG never launched, the estimated effect was 9.3 (95% confidence interval [CI] 1.04 to 17.5) additional monthly contacts per million individuals aged 15 years and older in February 2022 (an approximately 80.7% increase). Compared with the scenario in which neither PlayOLG nor private online gambling was allowed, the estimated effect was 20.8 (95% CI 8.5 to 33.2) additional monthly contacts per million individuals aged 15 years and older in September 2025 (an approximately 198.3% increase). Increases in gambling-related contacts occurred almost exclusively in adolescent boys and men aged 15 to 44 years. Compared with no private online gambling, and no private online gambling or PlayOLG, gambling-related contacts in adolescent boys and men aged 15 to 24 years were estimated to have increased by 144% and 337.8%, respectively. Interpretation: The introduction of online gambling and subsequent privatization with single-event sports betting were each associated with substantial increases in gambling-related helpline contacts, specifically in adolescent boys and young men, suggesting increased gambling problems, care-seeking, or both. This underscores the need to strengthen preventive measures and treatment access.