Importance: Following decades-long declines, significant increases in abortion rates have been reported in some jurisdictions from 2020 to 2023, but it is not yet known whether these trends are occurring in Canada. Objective: To assess abortion rates in Ontario from 2012 to 2022 and to examine trend changes associated with mifepristone availability, the COVID-19 pandemic, and postpandemic periods. Design, Setting, and Participants: This population-based interrupted time series cohort study examined all medication and procedural abortions provided in Ontario from January 1, 2012, to December 31, 2022, to females aged 15 to 44 years with provincial insurance coverage, identified using linked health administrative data that included records from practitioner billings, inpatient and outpatient hospital services, same-day surgeries, and outpatient prescription dispensations. Exposure: Availability of mifepristone regulated as a normal (ie, prescribed by an authorized prescriber without additional certification or registration and dispensed by a pharmacist) prescription medication (in November 2017) and the COVID-19 pandemic period (from March 2020 to December 2021). Main Outcomes and Measures: The main outcome was the abortion rate (number of abortions per 1000 females per year) overall and within age strata, using an interrupted time series design. Results: Of 422 867 medication and procedural abortions identified using data from health records of 225 540 reproductive-aged females (mean [SD] age, 28.5 [6.6] years), the abortion rate declined steadily from 15.6 abortions per year per 1000 females, aged 15 to 44 years, in 2012 to 12.3 in 2021 and then increased to 14.1 in 2022. When mifepristone was introduced in 2017 as a normal prescription medication, no immediate change in the abortion rate (−0.1 [95% CI, −0.7 to 0.8]) and a nonsignificant slope increase (0.6 [95% CI, −0.5 to 0.7]) were found. However, this trend resulted in an additional 1.5 (95% CI, 0.3-2.6) abortions per 1000 females by the first quarter of 2020 compared with premifepristone trends; rates increased more among those aged 15 to 19 years, less among those aged 35 to 44 years, and did not increase for those aged 25 to 29 years. During the pandemic period, abortion rates decreased by 1.2 (95% CI, −2.5 to −0.8), most pronounced among those aged 20 to 34 years. Compared with expected rates based on premifepristone trends, 5-year availability of normally prescribed mifepristone was associated with a rate difference of 1.9 (95% CI, 0.7-5.4) in 2022, with a greater increase among those aged 20 to 24 years (4.2 [95% CI, 1.5-9.0]) and no change among those aged 25 to 29 years (1.0 [95% CI, −1.7 to 6.2]). The increased abortion rate in 2022 was consistent with 5-year trends following normally prescribed mifepristone, although social forces potentially impacting international rates may have contributed. Conclusions and Relevance: This study found that, following longstanding declines, abortion rates in Ontario gradually increased with mifepristone availability in 2017 in Ontario. Following a pandemic-related decrease in rates (in 2020 and 2021), substantial increases in abortion rates reported elsewhere from 2020 to 2023 did not occur in Ontario as of 2022, suggesting that Ontario’s health services environment and Canada’s regulatory and policy approach to preserving reproductive health services may have helped stabilize abortion rates. Future research is needed to understand how sociocultural changes affecting abortion service use elsewhere may be affecting contraception access and use and thus abortion rates in Canada.