Childhood trauma has been shown to have detrimental consequences on mental health. It is unknown what impact childhood trauma may have on the early trajectory of serious mental illness (SMI). The purpose of this article is to estimate the baseline prevalence, perceived impact, and duration of trauma that occurred before the age of 18 years in youth at risk for SMI using a transdiagnostic approach. This study included 243 youths, ages 12 to 25 years (42 healthy controls, 43 non-help-seeking individuals [stage 0], 52 help-seeking youth experiencing distress and possibly mild symptoms of anxiety or depression [stage1a], and 108 youth demonstrating attenuated symptoms of an SMI such as bipolar disorder or psychosis [stage 1b]). Participants completed an adapted version of the Childhood Trauma and Abuse scale. There were high frequencies of reported trauma across all stages. Symptomatic individuals experienced more trauma and bullying. Stage 1b individuals reported more physical abuse. Stage 1b also indicated psychological bullying to have a longer duration and impact on their lives. Future work should aim to clarify the complex interrelations between trauma and risk of SMI.