Background: Mental illnesses are one of the health challenges that impose a heavy health burden on patients, families, and society at large due to a general disdain and lack of care in comparison to other illnesses. Objective: This study examined the general public’s knowledge and its relationship to attitudes and behaviors toward mental illnesses in Cameroon’s South West Region. Materials and Methods: This was a cross-sectional study with 400 respondents out of 1500 participants who were recruited via an online survey. The Mental Health Knowledge Schedule (MAKS), Community Attitudes Toward Mental Illness (CAMI), and Reported and Intended Behavior Scale (RIBS) were used to assess knowledge, attitudes, and behaviors toward mental illness, respectively. Sociodemographic information was also gathered. The cutoff points for low, medium and high scores were set at the 25th, 50th, and 75th percentiles of the MAKS, CAMI, and RIBS scores, respectively. Results: The survey was completed by 400 people out of the 1500 consenting participants recruited for the study. Approximately 72 percent of study participants had a high score for public stigma toward people with mental illness. In terms of knowledge, 32.1 percent had little knowledge of mental health, while 62.3 percent reported favorable attitudes toward people with mental illnesses. Tertiary education (β = 0.018, p 0.05), having a close relative/friend with mental illness (β = 0.029, p 0.05), and current or past history of mental illness (β = 0.079, p 0.05) all had a positive impact on knowledge of mental health after multivariate adjustments. Higher MAKS scores (β = 0.142, p 0.001), living in an urban area (β = 0.372, p 0.001), tertiary level of education (β = 0.812, p 0.001), having a close relative/friend with mental illness (β = 0.376, p 0.001), and having a current or past history of mental illness (β = 1.032, p 0.001) were associated with a lower likelihood of stigmatization against people with mental illness (higher CAMI scores). Higher MAKS scores (β = 2.120, p 0.001), higher CAMI scores (β = 1.043, p 0.001), having a close relative/friend with mental illness (β = 0.610, p 0.001), and having a current or past history of mental illness (β = 2.381, p 0.001) all had a significant impact on the positive behavior (higher RIBS scores) towards people with mental illness. Living in a rural area (β = -0.514, p 0.001) and being a non-healthcare worker (β = -0.245, p 0.05) were both associated with lower mental health knowledge. Conclusions: Our findings indicate that people who live in rural areas, non-healthcare workers, and people with a low level of education have poor knowledge, attitudes, and behaviors toward people with mental illness. To improve mental health knowledge, attitudes, and behaviors toward the mentally ill, these groups should be prioritized in health promotion interventions. Keywords: Serum lycopene, Lung cancer, Mortality risk, Cohort study