Obesity is a leading cause of preventable death among individuals with serious mental illness (SMI). A prior randomized controlled trial demonstrated the efficacy of a lifestyle style intervention tailored to this population; however, such interventions need to be adapted and tested for real-world settings. This study evaluated implementation interventions to support community mental health program staff to deliver an evidence-based lifestyle intervention to clients with obesity and SMI. In this cluster-randomized pilot trial, the standard arm combined multimodal training with organizational strategy meetings and the enhanced arm included all standard strategies plus performance coaching. Staff-coaches delivered a 6-month group-based lifestyle intervention to clients with SMI. Primary outcomes were changes in staff knowledge, self-efficacy, and fidelity scores for lifestyle intervention delivery. Linear mixed-effects modeling was used to analyze outcomes, addressing within-site clustering and within-participant longitudinal correlation of outcomes. Three sites were in the standard arm (7 staff-coaches); 5 sites in the enhanced arm (11 staff-coaches). All sites delivered all 26 modules of the lifestyle intervention. Staff-coaches highly rated the training strategy's acceptability, feasibility and appropriateness. Overall, mean knowledge score significantly increased pre-post by 5.5 (95% CI: 3.9, 7.1) and self-efficacy was unchanged; neither significantly differed between arms. Fidelity ratings remained stable over time and did not differ between arms. Clients with SMI achieved a mean 6-month weight loss of 3.8 kg (95% CI: 1.6, 6.1). Mental health staff delivering a lifestyle intervention was feasible using multicomponent implementation interventions, and preliminary results show weight reduction among clients with SMI. The addition of performance coaching did not significantly change outcomes. Future studies are needed to definitively determine the effect on client health outcomes.