Person-centered care has been gaining prominence in behavioral health care, and service planning has shifted towards “person-centered care planning” (PCCP), where individuals, in partnership with providers, identify life goals and interventions. A strong therapeutic alliance has been identified as key to a person-centered approach, but little is known about how the therapeutic relationship influences person-centered processes and outcomes. Using an explanatory sequential mixed methods design, this study investigated: (1) the association between the therapeutic alliance and PCCP, and (2) how the therapeutic relationship influences the process and outcomes of PCCP. Quantitative analyses found that a strong working alliance predicted greater personcenteredness. Qualitative analyses revealed two central themes: (1) the importance of connection, continuity, and calibration of the relationship to set the right conditions for PCCP, and (2) PCCP as a vehicle for engagement. Findings demonstrated that the therapeutic alliance is inextricably linked to the PCCP process, each influencing the other.