The external dacryocystorhinostomy (Ex-DCR) is a reliable but surgically challenging procedure to overcome a nasolacrimal duct obstruction (NLDO). The aim of this study is to describe the outcomes of a modified technique of lacrimal sac opening and to compare it with the conventional Ex-DCR. This comparative cohort study included adult patients undergoing Ex-DCR for primary acquired NLDO by or under supervision of one surgeon. Group 1 (conventional Ex-DCR including H-shaped incision, anastomosis of the anterior mucosal flap to nasal mucosal flap and resection of the posterior sac flap) was compared with Group 2 (modified Ex-DCR including excision of the medial lacrimal sac and anastomosis of remaining anterior sac flap to nasal mucosal flap). Outcome measures included the success rate (defined as complete symptom relief or patent irrigation after three months), reoperation rate, redo-free survival within five years, and occurrence of postoperative bleeding and postoperative infection. 138 patients were included. The success rates were 94.7% (54/57 cases) for Group 1 and 96.3% (78/81) for Group 2 (p-value = 0.658). The redo-rate was 5.3% (3/75) in Group 1 and 2.5% (2/81) in Group 2 (p = 0.331). Two-year redo-free survival probability estimates were 89.8% for Group 1 and 96.3% for Group 2, respectively. No complications occurred in Group 2, whereas in Group 1, one patient (1.8%) suffered from postoperative bleeding and one (1.8%) from postoperative infection. This study showed that our modified Ex-DCR technique is equally efficacious compared with the conventional Ex-DCR technique in adult patients with NLDO.