Introduction Involuntary psychiatric hospitalization occurs when someone with a serious mental disorder requires treatment without their consent. Trends vary globally, and currently, there is limited data on involuntary hospitalization in Canada. We examine involuntary hospitalization trends in British Columbia, Canada, and describe the social and clinical characteristics of people ages 15 and older who were involuntarily hospitalized between 2008/2009 and 2017/2018. Method We used population-based linked administrative data to examine and compare trends in involuntary and voluntary hospitalizations for mental and substance use disorders. We described patient characteristics (sex/gender, age, health authority, income, urbanity/rurality, and primary diagnosis) and tracked the count of involuntarily hospitalized people over time by diagnosis. Finally, we examined population-based prevalence over time by age and sex/gender. Results Involuntary hospitalizations among British Columbians ages 15 and older rose from 14,195 to 23,531 (65.7%) between 2008/2009 and 2017/2018. Apprehensions involving police increased from 3,502 to 8,009 (128.7%). Meanwhile, voluntary admissions remained relatively stable, with a minimal increase from 17,651 in 2008/2009 to 17,751 in 2017/2018 (0.5%). The most common diagnosis for involuntary patients in 2017/2018 was mood disorders (25.1%), followed by schizophrenia (22.3%), and substance use disorders (18.8%). From 2008/2009 to 2017/2018, the greatest increase was observed for substance use disorders (139%). Over time, population-based prevalence increased most rapidly among women ages 15–24 (162%) and men ages 15–34 (81%) and 85 and older (106%). Conclusion Findings highlight the need to strengthen the voluntary care system for mental health and substance use, especially for younger adults, and people who use substances. They also signal a need for closer examination of the use of involuntary treatment for substance use disorders, as well as further research exploring forces driving police involvement and its implications.