BACKGROUND: Psychological interventions targeting children with mild intellectual disability or borderline intellectual functioning (MID-BIF) are suggested to be effective in reducing their externalizing problem behavior, but less is known about the specific treatment processes that may be associated with these effects. AIMS: The current study investigated whether the treatment processes of observed treatment adherence (i.e., the degree to which a therapist sticks to the protocol of a treatment and provides the treatment as intended) and observed therapist alliance-building behavior (TA-BB; i.e., behavior contributing to the affective bond between the therapist and the client) predicted treatment outcomes in a group behavioral parent training combined with group child cognitive behavior therapy targeting externalizing problem behavior in children with MID-BIF. METHODS AND PROCEDURES: Seventy-two children (aged 9-18; Mage = 12.1) and their parents in The Netherlands received the intervention program. They reported on children's externalizing behavior, parenting practices and the parent-child relationship by questionnaires at pre-test and post-test, and the observed treatment processes were coded by audio tapes of therapeutic sessions. OUTCOMES AND RESULTS: The results showed high levels of both treatment adherence (M = 2.49; SD = 0.20; range 1 - 3) and TA-BB (M = 4.11; SD = 0.32; range 1 - 5). Additionally, repeated measures analyses revealed that levels of treatment adherence significantly predicted the improvement of the parent-child relationship (F(1, 66) = 5.37; p = .024) and that levels of TA-BB significantly predicted the decrease of parent reported externalizing problem behavior (F(1, 66) = 9.89; p = .002). CONCLUSIONS AND IMPLICATIONS: The current study suggested that optimal treatment processes are important for treatment outcomes in an intervention targeting children with MID-BIF.