Suicide is the second leading cause of death for people aged 10-34 years old. Limited research has documented extant heterogeneities in suicide across the life course and among diverse sociodemographic groups. There is also limited research on the influences of mental health utilization on suicidal trajectories across the life course. This study aims to: (1) identify racial/ethnic, sex, sexual orientation, socioeconomic status, and intersectional differences in suicidal trajectories among adolescents transitioning to adulthood; and (2) examine influences of mental health service utilization on disparities in suicidal trajectories. The study included 9421 respondents (M = 14.99 [SD = 1.61]) from Waves I-IV National Longitudinal Study of Adolescent to Adult Health (1994-2008). Latent class growth analyses were used to identify trajectories of suicidal ideation and suicide attempts. Multivariate multinomial logistic regression was used to examine the influences of mental health treatment and sociodemographic characteristics on suicidal trajectories. Three suicidal ideation (low-stable, high-decreasing, moderate-decreasing-increasing) and two suicide attempt (low-stable, moderate-decreasing) trajectories were identified. Compared with the low-stable trajectories, the risks of being in high-decreasing suicidal ideation trajectories were higher among females (AOR = 1.45, 95% CI 1.01-2.13) and sexual minorities (AOR = 1.82, 95% CI 1.21-2.74). Sexual minorities (AOR = 2.63, 95% CI 1.69-4.08) and low-SES adolescents (AOR = 1.79, 95% CI 1.08-2.98) were more likely to be in the moderate-decreasing suicide attempt group. Mental health service utilization predicted engagement in high-risk suicidal trajectories. Sociodemographic disparities in suicidal trajectories initiate early and persist over time. Individuals in high-risk trajectories received mental health treatment during adolescence. Suicide prevention should target vulnerable subpopulations and mental health service utilization in the early stage.