Background: Shared decision making in the emergency department (ED) can increase patient engagement for patients presenting with chest pain. However, little is known regarding which factors are associated with actual patient involvement in decision making or patients’ desired involvement in emergency care decisions. We examined which factors were associated with patients’ actual and desired involvement in decision making among ED chest pain patients. Methods: This is a secondary analysis of data from a randomized trial of a shared decision-making intervention in ED patients with low-risk chest pain. We evaluated the degree to which patients were involved in decision making using the OPTION-12 (observing patient involvement) scale and patients’ reported desire for involvement in decision making using the Control Preferences Scale (CPS). We measured the associations of patient factors with OPTION-12 and CPS scores using multivariable regression. Results: Of the 898 patients enrolled, mean (±SD) age was 51.5 (±11.4) years and 59% were female. Multivariable analysis revealed that only two factors were significantly associated with OPTION-12 scores: study site and use of the decision aid. OPTION-12 scores were 10.3 (standard error = 0.6) points higher for patients randomized to the decision aid compared to usual care (p < 0.001). Higher health literacy was associated with lower scores on the CPS, indicating greater desire for involvement (odds ratio = 0.91, p < 0.001). Conclusions: Patients’ reported desire for involvement in decision making was higher among those with higher health literacy. After study site and other potential confounding factors were adjusted for, only use of the decision aid was associated with observed patient involvement in decision making. As the science and practice of shared decision making in the ED moves toward implementation, high-fidelity integration of the decision aid into the flow of care will be necessary to realize desired outcomes.