Background. To determine if the use of theory, data and end-user perspectives to guide an adaptation of the Transdiagnostic Intervention for Sleep and Circadian Dysfunction (TranS-C) yields better outcomes and improves the "fit" of TranS-C to community mental health centers (CMHCs), relative to the standard version. Methods. Ten counties in California were cluster-randomized by county to Adapted or Standard TranS-C. Within each county, adults who exhibited sleep and circadian dysfunction and serious mental illness (SMI) were randomized to immediate TranS-C or Usual Care followed by Delayed Treatment with TranS-C (UC-DT). Facilitation was the implementation strategy. The participants were 93 CMHC providers who delivered TranS-C (Standard = 30; Adapted = 63) and 396 CMHC patients (Standard = 74; Adapted = 124; UC-DT = 198). Patient assessments were completed at pre-treatment, post-treatment, and six months after treatment (6FU). Provider assessments were completed at post-training, mid-treatment, and post-treatment. Results. TranS-C (combining Adapted and Standard), relative to UC-DT before delayed treatment with TranS-C, was associated with improvement from pre- to post-treatment in sleep disturbance ( b = -10.91, p < 0.001, d = -1.52), sleep-related impairment ( b = -9.52, p < 0.001, d = -1.06), sleep health composite ( b = 1.63, p < 0.001, d = 0.95), psychiatric symptoms ( b = -6.72, p < 0.001, d = -0.52), and overall functional impairment ( b = -5.12, p < 0.001, d = -0.71). TranS-C's benefits for functional impairment and psychiatric symptoms were mediated by improvements in sleep and circadian problems. Adapted versus Standard TranS-C did not differ on provider ratings of fit and better fit did not mediate the relation between TranS-C condition and patient outcome. Conclusions. TranS-C can be delivered by CMHC providers. Although Adapted and Standard TranS-C both fit the CMHC context, several advantages emerged for the adapted version. Trial registration: Clinicaltrials.gov identifier: NCT04154631. Registered on November 6, 2019. https://clinicaltrials.gov/ct2/show/NCT04154631.