Objective: To assess the feasibility and safety of the full-endoscopic trans-lumbosacral triangle approach for treating L5/S1 far-lateral lumbar disc herniation (FLLDH) and to evaluate its mid-to long-term clinical outcomes. Methods: A single-center retrospective observational study was conducted on 44 patients with L5/S1 far-lateral lumbar disc herniation. All patients underwent full-endoscopic trans-lumbosacral triangle approach surgery. Outcomes were assessed using the Visual Analog Scale (VAS) for low back and lower limb pain, the Oswestry Disability Index (ODI) for lumbar function, and the modified MacNab criteria for excellent/good rates. Radiographic follow-up included dynamic lumbar radiographs, thin-slice lumbar CT, and lumbar MRI. Results: The operative time for the 44 patients ranged from 45 to 73 minutes, with a mean of 58.0 ± 7.71 min. The follow-up period ranged from 35 to 46 months, with a median of 40 months. The low back pain VAS score significantly decreased from 5.41 ± 1.06 preoperatively to 1.57 ± 0.82 at 1-month postoperatively and further reduced to 0.93 ± 0.73 at the final follow-up. The leg pain VAS score decreased from 6.80 ± 1.00 preoperatively to 1.43 ± 0.50 at 1-month postoperatively and 0.84 ± 0.53 at the final follow-up. The ODI improved from 62.93 ± 14.22% preoperatively to 20.58 ± 12.56% at 1-month postoperatively and 6.99 ± 5.47% at the final follow-up. The differences in VAS and ODI scores at both 1-month and final follow-up were statistically significant compared to preoperative values (P < 0.05), and this difference remained statistically significant when compared the final follow-up to the 1-month postoperative assessment (P < 0.05). According to the modified MacNab criteria at the final follow-up, yielding an excellent/good rate of 93.2%. Postoperative thin-slice lumbar CT and MRI confirmed complete removal of the herniated nucleus pulposus, and dynamic lumbar radiographs showed no instability. Conclusion: The full-endoscopic trans-lumbosacral triangle approach demonstrates favorable mid-to long-term clinical outcomes in the treatment of L5/S1 far-lateral lumbar disc herniation.