Dysphagia is common after an open partial horizontal laryngectomy (OPHL). The mechanisms causing lower airways’ invasion and pharyngeal residue are unclear. The study aims to examine physio-pathological mechanisms affecting swallowing safety and efficiency after OPHL. Fifteen patients who underwent an OPHL type IIa with arytenoid resection were recruited. Videofluoroscopic examination of swallowing was performed. Ten spatial, temporal, and scalar parameters were analyzed. Swallowing safety and efficiency were assessed through the Dynamic Imaging Grade of Swallowing Toxicity (DIGEST) scale. Swallowing was considered unsafe or inefficient for a DIGEST safety or efficiency grade ≥2, respectively. Videofluoroscopic measurements were compared between safe vs. unsafe swallowers, and efficient vs. inefficient swallowers. Seven patients (46.7%) showed unsafe swallowing and 6 patients (40%) inefficient swallowing. Unsafe swallowers had worse laryngeal closure (p = 0.021). Inefficient swallowers presented a longer pharyngeal transit time (p = 0.008), a reduced pharyngoesophageal segment opening lateral (p = 0.008), and a worse tongue base retraction (p = 0.018 with solids and p = 0.049 with semisolids). In conclusion, swallowing safety was affected by incomplete laryngeal closure, while swallowing efficiency was affected by increased pharyngeal transit time, reduced upper esophageal sphincter opening, and incomplete tongue base retraction. The identified physio-pathological mechanisms could represent targets for rehabilitative and surgical approaches in patients with dysphagia after OPHL.