PurposeOne-to-one peer supporters called isPO onco-guides (isPO OGs) are an integral part of the new German psycho-oncological form of care ‘integrated, cross-sectoral Psycho-Oncology’ (isPO), additionally to professional care. The isPO OGs are cancer survivors with experiential knowledge, offering information on local support services and answering questions ‘all around cancer’ to newly diagnosed cancer patients. We aimed to evaluate the isPO OG service from three perspectives: patients, isPO OGs, and professional service providers.MethodsA mixed-methods approach was pursued. We conducted interviews and focus groups with the three person groups, and applied qualitative content analysis on the reported resources, processes and outcomes regarding the isPO OG service. Relations with patients’ utilisation and isPO OGs’ work satisfaction were identified with regression and correlation analyses of questionnaire and isPO care data. We compared isPO care networks (CN) withX2-tests or ANOVA. Qualitative and quantitative results were integrated during interpretation phase.ResultsQualitatively, the three person groups agreed on the benefits of the isPO OG service. The implementation’s maturity differed between the CN concerning established processes and resource availability. Attitudes of professional service providers appeared to be crucial for patients’ utilisation of the isPO OG service. Quantitative results emphasised the differences between the CN.ConclusionBeyond differences in the CN, the isPO OG service has two psychosocial benefits: providing relevant, reliable, and understandable information; and offering the encouraging example that surviving and living with cancer is possible.Trial registrationThe study was registered in the German Clinical Trials Register (No. DRKS00015326) on 30.10.2018.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00432-023-04951-4.