Objectives: Given the importance of supporting cancer patients to quit smoking, we sought to ascertain cancer care clinicians' beliefs and practices regarding providing smoking cessation brief interventions. Methods:We used a cross-sectional sequential explanatory mixed method design, including a survey of multidisciplinary cancer care clinicians and semistructured interviews. Results: One hundred and sixty-five cancer care clinicians completed the survey and 21 participated in interviews. Over half of survey respondents (53%) said they do not regularly undertake smoking cessation brief interventions and 40% rarely or never advise quitting. Nonmetropolitan clinicians were more likely to discuss medication options and refer to the Quitline. Physicians were more likely to do brief interventions with patients and radiation therapists were least likely. Barriers were lack of training and experience, lack of knowledge of the Quitline referral process, lack of role clarity, lack of resources and systems, and perceived psychological ramifications of cancer for patients. Conclusion: There is a need to upskill cancer clinicians and improve systems to provide smoking cessation brief interventions as part of routine clinical practice. All cancer care clinicians should complete brief intervention smoking cessation training relevant to the cancer context, including making referrals to Quitline, and be supported by systems to record and follow-up care.