Grounded in the Integrated Promoting Action on Research Implementation in Health Services (i-PARIHS) framework, we investigated the Train-the-Trainer (TTT) to expand access to evidence-based psychological treatments (EBPTs) in community mental health centers (CMHCs), focusing on the Transdiagnostic Intervention for Sleep and Circadian Dysfunction (TSC). Eight Californian counties were cluster-randomized to Standard TSC or an adapted version designed to improve the "fit" of TSC to CMHCs. University-based trainers trained CMHC providers ("Generation 1 providers") in either Adapted or Standard TSC. These trained providers were then trained to become local CMHC trainers ("Generation 1 trainers"), who then trained a new cohort of providers ("Generation 2 providers") in TSC. Within each county, patients diagnosed with serious mental illness (SMI) were randomized to receive either immediate TSC or usual care and delayed treatment with TSC (UC-DT) from the Generation 2 providers ("Generation 2 patients"). This study focused on 53 Generation 2 providers (Adapted TSC = 47; Standard TSC = 6), and 143 Generation 2 patients (Adapted TSC = 127; Standard TSC = 16) (the larger Adapted sample was driven by recruitment, perhaps reflecting preference for the "fitted" approach). Patient assessments were conducted pre-treatment, post-treatment, and six-month follow-up (6FU). Provider assessments occurred after completing TSC training and post-treatment for each patient treated. Combining Adapted and Standard, TSC was associated with improvements for Generation 2 patients from pre- to post-treatment in sleep disturbance ( < 0.001, -0.90), sleep-related impairment ( = 0.001, -0.69), psychiatric symptoms ( = 0.002, = -0.48), and functional impairment ( = 0.002, = -0.54), relative to UC-DT. The effects of sleep disturbance and impairment on the relationship between treatment condition (TSC vs. UC-DT) and psychiatric symptoms and functional impairment were significant. Higher provider perception of TSC fit predicted improvements in selected patient outcomes. TSC can be delivered by CMHC providers trained by local CMHC trainers with strong outcomes. These data contribute to the dearth of evidence for TTT collected from locally trained providers and from patients treated by local CMHC trainers. Clinicaltrials.gov identifier: NCT05805657. Registered on March 10, 2023. https://clinicaltrials.gov/ct2/show/NCT05805657.