Background: This study was conducted to examine whether a longitudinal advance care planning (ACP) intervention facilitates concordance between the preferred and received life-sustaining treatments (LSTs) of terminally ill patients with cancer and improves quality of life (QoL), anxiety symptoms, and depressive symptoms during the dying process. Patients and Methods: Of 795 terminally ill patients with cancer from a medical center in Taiwan, 460 were recruited and randomly assigned 1:1 to the experimental and control arms. The experimental arm received an interactive ACP intervention tailored to participants’ readiness to engage in this process. The control arm received symptom management education. Group allocation was concealed, data collectors were blinded, and treatment fidelity was monitored. Outcome measures included 6 preferred and received LSTs, QoL, anxiety symptoms, and depressive symptoms. Intervention effectiveness was evaluated by intention-to-treat analysis. Results: Participants providing data had died through December 2017. The 2 study arms did not differ significantly in concordance between the 6 preferred and received LSTs examined (odds ratios, 0.966 [95% CI, 0.653–1.428] and 1.107 [95% CI, 0.690–1.775]). Participants who received the ACP intervention had significantly fewer anxiety symptoms (β, −0.583; 95% CI, −0.977 to −0.189; P= .004) and depressive symptoms (β, −0.533; 95% CI, −1.036 to −0.030; P= .038) compared with those in the control arm, but QoL did not differ. Conclusions: Our ACP intervention facilitated participants’ psychological adjustment to the end-of-life (EoL) care decision-making process, but neither improved QoL nor facilitated EoL care honoring their wishes. The inability of our intervention to improve concordance may have been due to the family power to override patients’ wishes in deeply Confucian doctrine–influenced societies such as Taiwan. Nevertheless, our findings reassure healthcare professionals that such an ACP intervention does not harm but improves the psychological well-being of terminally ill patients with cancer, thereby encouraging physicians to discuss EoL care preferences with patients and involve family caregivers in EoL care decision-making to eventually lead to patient value–concordant EoL cancer care.