The objective of this mixed methods review was to examine the effectiveness and family experiences of interventions promoting partnerships between families and the multidisciplinary health care team in pediatric and neonatal intensive care units. Hospitalization of infants and children in neonatal intensive care units and pediatric intensive care units has a significant effect on their families, including increased stress, anxiety, and depression. Available evidence syntheses focused on specific family-centered care, but not on partnership, which is another aspect that may improve families' outcomes and experiences. This review considered studies that focused on effectiveness or family experiences of interventions by health professionals in partnership with families of infants or children hospitalized in an intensive care unit. For the quantitative component of the review, the type of intervention was a partnership between the health care team and the family, and focused on outcomes of stress, anxiety, depression, quality of life, attachment, or satisfaction with family-centered care. For the qualitative component, the phenomenon of interest was family experiences of interventions that included collaboration and partnering with the health care team in the pediatric or neonatal intensive care unit. Quantitative, qualitative, and mixed methods studies, published from 2000 to August 2022 in English or French, were eligible for inclusion. The JBI methodology for convergent segregated mixed methods systematic reviews was followed using the standardized JBI critical appraisal and data extraction tools. Ten databases were searched in December 2019 and again in August 2022. Study selection, critical appraisal, and data extraction were performed by 2 reviewers independently. Findings of quantitative studies were statistically pooled through meta-analysis and those that could not be pooled were reported narratively. Qualitative studies were pooled through meta-synthesis. This review included 6 qualitative and 42 quantitative studies. The methodological quality varied, and all studies were included regardless of methodological quality. Meta-analyses showed improvements in anxiety, satisfaction with family-centered care, and stress, yet no conclusive effects in attachment and depression. These results should be interpreted with caution due to high heterogeneity. Qualitative analysis resulted in 2 synthesized findings: "Interventions that incorporate partnerships between families and the health care team can improve the family's experience and capacity to care for the child" and "Having a child in intensive care can be an experience of significant impact for families." Integration of quantitative and qualitative evidence revealed some congruence between findings; however, the paucity of qualitative evidence minimized the depth of this integration. Partnership interventions can have a positive impact on parents of children in intensive care units, with improvements reported in stress, anxiety, and satisfaction with family-centered care. PROSPERO CRD42019137834. A Chinese-language version of the abstract of this review is available at http://links.lww.com/SRX/A50 . A French-language version of the abstract of this review is available at http://links.lww.com/SRX/A51 .