Background: Although LGBT+ people show high rates of utilization of mental health services, psychotherapeutic care seems to fail to meet their specific needs. This is mainly due to professionals’ lack of knowledge about the unique aspects of life development and mental health processes of LGBT+ people, as well as the inadvertent presence of prejudice and negative attitudes in psychotherapists. Aim: This study sought to establish the relationship between beliefs, level of experience, and competencies for working with LGBT people with the presence of prejudice in psychology students and psychotherapist practitioners. Method: 50 undergraduate students and 380 professional psychologists from Chile answered an online survey that was composed of scales that sought to assess prejudices, beliefs about homosexuality, bisexuality, and transsexuality and the development of clinical competencies. Results: Participants held more psychosocial beliefs about the origin of homosexuality and bisexuality when compared with transsexuality. Low positive correlations were observed between years of clinical practice and psychological beliefs, and low negative correlations between biological and psychological beliefs and level of clinical preparedness for working with LGBT people. Higher levels of beliefs about homosexuality, bisexuality, and transsexuality were associated with more prejudiced attitudes. Conclusions: This research provided evidence in favor of the relationship between prejudice and beliefs in both experienced clinicians and psychology students. It also showed that the higher the level of clinical competencies for working with patients of sexual and gender diversity, the lower the presence of prejudice.