PURPOSE: This study aimed to characterize two developmental cohorts of transgender and nonbinary youth enrolled in the Trans Youth Care Network Study and describe their gender identity-related milestones and baseline mental health and psychosocial functioning. METHODS: Trans Youth Care participants were recruited from four pediatric academic medical centers in the U.S. before initiating medical treatment for gender dysphoria either with gonadotropin-releasing hormone agonists (GnRHa) or gender-affirming hormones (GAH). GnRHa cohort data were collected from youth and a parent; GAH cohort data were collected from youth only. RESULTS: A total of 95 youth were enrolled in the GnRHa cohort. Mean age was 11.22 years (standard deviation = 1.46), and the majority were white (52.6%) and designated male at birth (51.6%). Elevated depression symptoms were endorsed by 28.6% of GnRHa cohort youth, and 22.1% endorsed clinically significant anxiety. Approximately one fourth (23.6%) endorsed lifetime suicidal ideation, with 7.9% reporting a past suicide attempt. A total of 316 youth were enrolled in the GAH cohort. The mean age was 16.0 years (standard deviation = 1.88), and the majority were white (62%) and designated female at birth (64.9%). Elevated depression symptoms were endorsed by 51.3% of the GAH cohort, and 57.3% endorsed clinically significant anxiety. Two-thirds (66.6%) endorsed lifetime suicidal ideation, with 24.6% reporting a past suicide attempt. Life satisfaction was lower among both cohorts compared with population-based norms. CONCLUSIONS: GnRHa cohort youth appear to be functioning better from a psychosocial standpoint than GAH cohort youth, pointing to possible benefits of accessing gender-affirming treatment earlier in life.