Despite increasingly refined tools for identifying individuals at clinical high-risk for psychosis (CHR-P), less is known about the effectiveness of CHR-P interventions. The significant clinical heterogeneity among CHR-P individuals suggests that interventions may need to be personalized during this emerging illness phase. We examined longitudinal trajectories within-persons during treatment to investigate whether baseline factors predict symptomatic and functional outcomes. A total of 36 CHR-P individuals were rated on attenuated positive symptoms and functioning at baseline and each week during CHR-P step-based treatment. Linear mixed-effects models revealed that attenuated positive symptoms decreased during the study period, while functioning did not significantly change. When examining baseline predictors, a significant group-by-time interaction emerged whereby CHR-P individuals with more psychiatric comorbidities at baseline (indicating greater clinical complexity) improved in functioning during the study period relative to CHR-P individuals with fewer comorbidities. Individual differences in clinical complexity may predict functional response during the early phases of CHR-P treatment.