People with serious mental illness (SMI) have identified barriers to engaging in behavioral weight management interventions (BWMIs). We assessed whether BWMIs that addressed these barriers were more effective. First, we systematically reviewed qualitative literature and used a thematic analysis to identify the characteristics of BWMIs that promote engagement for adults with SMI. Second, we systematically reviewed randomized controlled trials (RCTs) of BWMIs in adults with SMI. Data on the characteristics that promoted engagement and weight outcomes were extracted. We then used a crisp-set qualitative comparative analysis (CsQCA) to identify which characteristics were associated with weight loss. For the qualitative review, 20 studies in 515 people with SMI were analyzed and nine characteristics were reported to promote engagement in BWMIs. For the systematic review, 34 RCTs testing 36 interventions in 4305 participants were included. The active interventions resulted in more weight loss (mean = −4.37 to +1 kg at 6 weeks to 18 months follow-up) compared with controls (−1.64 to +3.08 kg). The CsQCA showed BWMIs that offered regular contact, tools to support enactment, and tailored materials were associated with effectiveness. As these are all supplementary strategies, it may be possible to augment BWMIs available for the general population to engage people with SMI.