Background: Although advance care planning discussions are increasingly accepted worldwide, their ideal timing is uncertain and cultural factors may pertain. Aim: To evaluate timing and factors affecting initiation of advance care planning discussions for adult patients in Japan and Taiwan. Design: Mixed-methods questionnaire survey to quantitatively determine percentages of patients willing to initiate advance care planning discussions at four stages of illness trajectory ranging from healthy to undeniably ill, and to identify qualitative perceptions underlying preferred timing. Setting/participants: Patients aged 40–75 years visiting outpatient departments at four Japanese and two Taiwanese hospitals were randomly recruited. Results: Overall (of 700 respondents), 72% (of 365) in Japan and 84% (of 335) in Taiwan (p < 0.001) accepted discussion before illness. In Japan, factors associated with willingness before illness were younger age and rejection of life-sustaining treatments; in Taiwan, older age, stronger social support, and rejection of life-sustaining treatments. Four main categories of attitudes were extracted: the most common welcomed discussion as a wise precaution, responses in this first category outnumbered preference for postponement of discussion until imminent end of life, acceptance of the universal inevitability of death, and preference for discussion at healthcare providers’ initiative. Conclusion: The majority of patients are willing to begin discussion before their health is severely compromised; about one out of five patients are unwilling to begin until clearly facing death. To promote advance care planning, healthcare providers must be mindful of patients’ preferences and factors associated with acceptance and reluctance to initiate advance care planning.