Purpose To study the long‐term outcome of deep sclerectomy with and without mitomycin‐C (MMC) in patients with normal‐tension glaucoma (NTG). Methods We prospectively analysed consecutive patients randomized to surgery performed either with (MMC group) or without (non‐MMC) MMC. Surgery was considered totally successful if, after surgery, the preoperative intra‐ocular pressure (IOP) level was reduced by 25% without medication, and a qualified success if medication was required to achieve the same limits. Results A total of 37 patients were enrolled, 15 in the MMC and 22 in the non‐MMC group. The median (range) follow‐up was 7.9 (1.0–9.0) years, with a drop‐out of three (8%) patients. The preoperative IOP was 15 (11–21) mmHg in the MMC and 15 (10–19) mmHg in the non‐MMC group. At the last 6‐ to 9‐year follow‐up, IOP was significantly reduced to 9 (2–13) mmHg (p = 0.002) and 10 (5–13) mmHg (p < 0.001). The overall (groups combined) complete and qualified success rates were 50% and 71%, with no significant difference between groups (p = 0.48 and p = 0.25). Goniopuncture was performed in 87% and 100% of eyes in the MMC and non‐MMC groups (p = 0.14). Needling with MMC injection was performed 0 (0–1) times in the MMC group and 0.5 (0–4) times in the non‐MMC group (p = 0.056). We encountered no cases of hyphema, shallow anterior chamber, hypotony maculopathy, choroidal effusion, late bleb leakage, blebitis, endophthalmitis or malignant glaucoma. Conclusion In NTG, long‐term significant IOP reduction can be achieved with deep sclerectomy with a low incidence of sight‐threatening complications.