Parents can play a vital role in shaping teenagers’ sexual attitudes, behavior, and contraceptive use through communication, however, less is known about how to modify parent–adolescent communication among youth with mental health problems. The impact of a family-based sexual risk prevention intervention on both observational and self-report of parent–adolescent sexual communication was examined at 12 months among adolescents with mental health problems. Of the 721 parent–adolescent dyads recruited for the study, 167 videotapes of sexual discussions between parents and adolescent were coded for the family-based intervention and 191 videotapes for the active comparison. Longitudinal analyses examined differences between conditions (family-based vs. comparison) in self-reported and observed parent–adolescent sexual discussions and also examined the impact of gender on intervention response. More parent I-statements, healthier parent body language, and fewer adolescent Negative Vocalizations were detected for family-based intervention participants 12 months after participating in the brief intervention (11 h of total intervention time) relative to those in the comparison condition. Parents in the family-based intervention also self-reported better sexual communication at 12 months. The current study provides supporting evidence that a relatively brief family-based intervention was successful at addressing parent–adolescent sexual communication among a mental health sample.