Background/Objectives: To evaluate the safety and efficacy of posterior scleral application (a modified technique) of antimetabolite mitomycin C (MMC)-soaked sponge in trabeculectomy for patients with glaucoma. Subjects/Methods: A total of 101 patients (115 eyes) with glaucoma, aged 12–83 years who underwent trabeculectomy using a modified MMC-soaked sponge placement method were enrolled in this observational study. The sponge was placed vertically and posteriorly with the long side perpendicular to the limbus. The MMC concentration was 0.2–0.5 mg/ml and the exposure time was 1–5 min. The intraocular pressure (IOP), best-corrected visual acuity, and the number of hypotensive medications taken at baseline and at the final visit were recorded. Complications, interventions required, and bleb morphology were recorded postoperatively. Bleb morphology, including height, extent, vascularity, Seidel test, and width, was graded using the Indiana Bleb Appearance Grading Scale. The primary outcome was the safety of trabeculectomy including complications and bleb morphology. The secondary outcome was the success rate of trabeculectomy. Results: The median follow-up period was 28 months. At the final follow-up, the qualified (cumulative) success rate was 93.0% and the complete success rate was 60.0%. No bleb-related complication was observed. The mean height was 0.6 ± 0.9. The mean extent was 1.2 ± 0.4. The mean vascularity was 2.4 ± 0.9. All Seidel tests were negative. The mean width was 0.8 ± 0.4. Conclusions: Trabeculectomy with the long side of the MMC-soaked sponge placed perpendicular to the corneal limbus to maximise the exposure area is safe and effective.