High-grade gliomas (HGG) are serious primary brain tumors that may prevent the patient from functioning normally in social, emotional and cognitive respect. Often the partner's role will convert to that of informal caregiver. Consequently, they may experience significant stress and reductions in caregiver mastery, negatively affecting their health-related quality of life (HRQOL). We aimed at (1) determining factors that impact HRQOL and mastery of caregivers of HGG patients, and (2) investigate if a structured intervention consisting of psychoeducation and cognitive behavioral therapy leads to improvements in the mental component of HRQOL and mastery of caregivers. Fifty-six patient-caregiver dyads were randomly assigned to the intervention group or the care as usual group. The intervention program consisted of six one-hour sessions with a psychologist. Participants completed questionnaires concerning their perceptions of the patients' HRQOL (SF-36), neurological functioning (BN20), and cognitive functioning (MOS), and concerning their own HRQOL (SF-36) and feelings of caregiver mastery (CMS) both at baseline (i.e. before randomization) and every 2 months thereafter until 8 months later, five times in total. Patients' HRQOL and neurological functioning were found to be related to HRQOL and feelings of mastery of the informal caregiver at baseline. The intervention helped caregivers in maintaining a stable level of HRQOL and improved feelings of mastery over an 8 month period. Our findings suggest that informal caregivers can benefit from a psychological intervention as it is a helpful tool in maintaining a stable level of mental functioning and caregiver mastery.