This study examined implicit and explicit measures of bias toward mental illness among people with different levels of mental health training, and investigated the influence of stigma on clinically-relevant decision-making. Participants (N = 1539) comprised of (1) mental health professionals and clinical graduate students, (2) other health care/social services specialists, (3) undergraduate students, and (4) the general public self-reported their attitudes toward people with mental illness, and completed implicit measures to assess mental illness evaluations that exist outside of awareness or control. In addition, participants predicted patient prognoses and assigned diagnoses after clinical vignettes. Compared with people without mental health training, individuals with mental health training demonstrated more positive implicit and explicit evaluations of people with mental illness. Further, explicit (but not implicit) biases predicted more negative patient prognoses, but implicit (and not explicit) biases predicted over-diagnosis, underscoring the value of using both implicit and explicit measures.