Background: Nonadherence to NCCN Guidelines during time from surgery to postoperative radiotherapy (S-PORT) can alter survival outcomes in head and neck squamous cell carcinomna (HNSCC). There is a need to validate this impact in an underserved urban population and to understand risk factors and reasons for delay. We sought to investigate the impact of delayed PORT with outcomes of overall survival (OS) in HNSCC, to analyze predictive factors of delayed PORT, and to identify reasons for delay. Methods: We conducted a retrospective cohort study in an urban, community-based academic center. A total of 184 patients with primary HNSCC were identified through the Montefiore Medical Center cancer registry who had been treated between March 1, 2005, and March 8, 2017, and met the inclusion and exclusion criteria. The primary exposure was S-PORT. OS, recurrence, and risk factors and reasons for treatment delay were the main outcomes and measures. Results: Among 184 patients with HNSCC treated with PORT, the median S-PORT was 48.5 days (interquartile range, 41–67 days). The S-PORT threshold that optimally differentiated worse OS outcomes was >50 days (46.7% of our cohort; n=86). Independent of other relevant factors, patients with HNSCC and S-PORT >50 days had worse OS (hazard ratio, 2.30; 95% CI, 1.34–3.95) and greater recurrence (odds ratio, 3.51; 95% CI, 1.31–9.39). Predictors of delayed S-PORT included being underweight or obese, prolonged postoperative length of stay, and age >70 years. The most frequent reasons for PORT delay were complications related to surgery (22.09%), unrelated medical comorbidities (18.60%), and nonadherence/missed appointments (6.98%). Conclusions: Delayed PORT beyond 50 days after surgery was associated with decreased OS and greater recurrence. Identification of predictive factors and reasons for treatment delay helps to target at-risk patients and facilitates interventions in underserved populations.