Loss of vision is a feared consequence of idiopathic intracranial hypertension (IIH). Optic nerve sheath fenestration (ONSF) may be an effective surgical approach to protect visual function in medically refractory IIH. In this study, we evaluate the impact of unilateral superomedial transconjunctival ONSF on bilateral visual outcome using a comprehensive follow-up program. A retrospective chart review of IIH patients who underwent unilateral ONSF between January 2016 and March 2021 was conducted. Patients fulfilling the revised Friedman criteria for IIH and who had exclusively received ONSF as a surgical treatment were included. Main outcomes were visual acuity (VA); perimetric mean deviation (PMD); papilledema grade; and optic nerve head elevation (max-ONHE) 1 week, 2 weeks, and 1, 3, and 6 months after surgery. VA (p < 0.05), PMD (p < 0.05), papilledema grade (p < 0.01), and maxOHNE (p < 0.001) were improved after 6 months on both the operated and non-operated eye. Prolonged surgical delay impedes PMD improvement (r = −0.78, p < 0.01), and an increasing opening pressure initiates a greater ganglion cell loss (r = −0.79, p < 0.01). In this small case series, we demonstrate that unilateral superonasal transconjunctival ONSF is a safe procedure with an effect on both eyes. Optic nerve head elevation and PMD are feasible biomarkers for assessing early treatment efficacy after ONSF.