<p>The aim of this study was to investigate sociodemographic characteristics related to mental health symptoms among a national sample of adolescents and young adults between the ages of 16 and 30 in Canada. Data from the Canadian Study of Adolescent Health Behavior (<i>N</i> = 2,731), a non-probability sample recruited via social media, were analyzed. Mental health symptoms (anxiety, non-suicidal self-injury, suicidal ideation, suicide attempt, and co-occurring mental health symptoms) were examined for association with sociodemographic characteristics via logistic and Poisson regression analyses. Data were collected from 2,731 adolescents and young adults recruited over social media and completed a survey that took 30&#xa0;min to complete online. In our sample, 68% of participants had at least one symptom of poor mental health, 51% of participants had an anxiety score indicative of a clinically relevant anxiety disorder, 37% screened positive for depression, 26% experienced suicidal ideation, and 3% had a suicide attempt. Findings suggest that mental health is worse among the younger groups of adolescents and young adults and that certain groups including girls/women, lesbian, gay, bisexual, transgender, queer/questioning (LGBTQ+), racialized, and Indigenous groups have higher rates of mental health symptoms and co-occurring mental health symptoms. Health care and public health professionals should be aware of the higher rates of poor mental health in adolescents and young adults in Canada to improve service delivery and mental health awareness. Future research should prioritize longitudinal designs to assess the temporal order of mental health conditions.</p>