Boarding, the period in which a patient spends in the emergency department (ED) before admission, may be hazardous to critically ill patients, particularly the elderly. This study investigated the associations of boarding with hospital course, prognosis, and medical expenditure in older patients. From January 2019 to December 2021, the medical records of older patients (age ≥ 65) visiting the ED of a tertiary referral hospital who were admitted to the medical intensive care unit (ICU) were retrospectively reviewed. Eligible patients were categorized into two groups according to boarding time with a cutoff set at 6 h. Primary outcomes were in-hospital mortality, ICU/hospital length of stay, and total/average hospitalization cost. Subgroup analyses considered age and disease type. Among 1318 ICU admissions from the ED, 36% were subjected to boarding for over 6 h. Prolonged boarding had a longer ICU (8.9 ± 8.8 vs. 11.2 ± 12.2 days, P < .001) and hospital (17.8 ± 20.1 vs. 22.8 ± 23.0 days, P < .001) stay, higher treatment cost (10.4 ± 13.9 vs. 13.2 ± 16.5 thousands of USD, P = .001), and hospital mortality (19% vs. 25% P = .020). Multivariate regression analysis showed a longer ICU stay in patients aged 65–79 (8.3 ± 8.4 vs. 11.8 ± 14.2 days, P < .001) and cardiology patients (6.9 ± 8.4 vs. 8.8 ± 9.7 days, P = .001). Besides, the treatment cost was also higher for both groups (10.4 ± 14.6 vs. 13.7 ± 17.7 thousands of USD, P = .004 and 8.4 ± 14.0 vs. 11.7 ± 16.6 thousands of USD, P < .001, respectively). Extended ED boarding for critically ill medical patients over 65 years old was associated with negative outcomes, including longer ICU/hospital stays, higher treatment costs, and hospital mortality. What is already known on this topic—Prolonged boarding in the ED may lead to negative outcomes. What this study adds—We focused on the elderly population to investigate the associations of ED boarding with hospital courses and healthcare expenses. Patients who experienced prolonged boarding before admission to the ICU due to cardiovascular diseases and those between the ages of 65 and 80 tended to have longer hospitalizations, lengthier ICU stays, and higher healthcare costs. How this study might affect research, practice or policy—As the phenomenon of global aging perpetuates, healthcare providers should implement strategies to reduce boarding times. This study highlights the importance of effectively allocating medical resources to improve healthcare quality and outcomes for this vulnerable population. Future Research Questions 1) Extending Findings to Non-Critical Older Patients—It remains unclear whether the adverse outcomes resulting from prolonged boarding can be extended to non-critical older patients. This investigation is essential for determining the universality of the impact of prolonged boarding on patients with varying degrees of severity. 2) Focus on Specific Diseases—Subsequent research efforts may be warranted to concentrate on specific diseases rather than broad diagnostic categories. 3) Enhancing Resource Allocation for Older Critical Patients—It may be necessary to develop new strategies for more effective allocation of healthcare resources to the vulnerable population. Further research is required to identify the specific factors leading to delayed ICU admission. Bullet Points Outlining the Main Message 1) Prolonged boarding in the ED for critically ill medical patients over 65 years old correlates with negative outcomes, including longer ICU/hospital stays, higher treatment costs, and hospital mortality. 2) In the relatively young population (i.e. aged between 65 and 80) and those admitted to the ICU due to cardiovascular diseases, our findings reveal that prolonged boarding is associated with longer hospitalization, extended ICU stays, and higher healthcare costs. 3) We may need to effectively allocate intensive care resources for older patients and specific populations.