This study aimed to evaluate the long-term surgical outcomes of trabeculectomy with mitomycin C (MMC) and identify factors associated with surgical failure in Thai patients with primary and secondary glaucoma. This retrospective study included 421 eyes from 397 patients with glaucoma who underwent trabeculectomy with MMC from January 1, 2012, to December 31, 2015. The patients' demographics, baseline characteristics, ocular parameters, and postoperative outcomes were analyzed. Surgical success was classified into complete success (IOP = 4-21 mmHg without anti-glaucoma medication), qualified success (IOP = 4-21 mmHg with medication), and failure (IOP >21 mmHg or <4 mmHg). Factors associated with surgical failure were assessed using Cox regression analysis. The cumulative probability of complete or qualified success at 8 years was 65.9% (95% CI: 59.9%, 71.9%); however, a significantly higher success rate was observed in the primary glaucoma group (75.1%) than in the secondary glaucoma group (47.4%) (p < 0.001). The secondary glaucoma group, particularly those with secondary closed-angle glaucoma, had the lowest success rates. Preoperative IOP, number of anti-glaucoma medications, and visual acuity were significantly different between the two groups. Etiologic factors associated with surgical failure included pseudo-exfoliation glaucoma (adjusted HR 3.23), neovascular glaucoma (adjusted HR 2.87), and secondary angle closure glaucoma (adjusted HR 2.62). Late complications were more prevalent in patients with secondary glaucoma than in those with primary glaucoma and included decreased visual acuity (5.43% vs 10.29%), hypotony (0.26% vs 1.14%), corneal decompensation (1.82% vs 3.40%), and bleb-related infection (0.19%). Trabeculectomy with MMC remains an effective surgical option for primary glaucoma, demonstrating better long-term outcomes compared to its outcomes in patients with secondary glaucoma. Risk factors for surgical failure include specific glaucoma etiologies. These findings would personalize management strategies to optimize surgical approaches based on predictive factors for surgical success.