Neuroprotective approaches comprising different mechanisms to counteract the noxious effects of excitotoxicity and oxidative stress need validation and detailed characterization. Although S-allylcysteine (SAC) is a natural compound exhibiting a broad spectrum of protective effects characterized by antioxidant, anti-inflammatory, and neuromodulatory actions, the mechanisms underlying its protective role on neuronal cell damage triggered by early excitotoxic insults remain elusive. In this study, we evaluated if the preconditioning or the post-treatment of isolated rat cortical slices with SAC (100 µM) can ameliorate the toxic effects induced by the excitotoxic metabolite quinolinic acid (QUIN, 100 µM), and whether this protective response involves the early display of specific antioxidant and neuroprotective signals. For this purpose, cell viability/mitochondrial reductive capacity, lipid peroxidation, levels of reduced and oxidized glutathione (GSH and GSSG, respectively), the rate of cell damage, the NF-E2-related factor 2/antioxidant response element (Nrf2/ARE) binding activity, heme oxygenase 1 (HO-1) regulation, extracellular signal-regulated kinase (ERK1/2) phosphorylation, and the levels of tumor necrosis factor-alpha (TNF-α) and the neurotrophin brain-derived neurotrophic factor (BDNF) were all estimated in tissue slices exposed to SAC and/or QUIN. The incubation of slices with QUIN augmented all toxic endpoints, whereas the addition of SAC prevented and/or recovered all toxic effects of QUIN, exhibiting better results when administered 60 min before the toxin and demonstrating protective and antioxidant properties. The early stimulation of Nrf2/ARE binding activity, the upregulation of HO-1, the ERK1/2 phosphorylation and the preservation of BDNF tissue levels by SAC demonstrate that this molecule displays a wide range of early protective signals by triggering orchestrated antioxidant responses and neuroprotective strategies. The relevance of the characterization of these mechanisms lies in the confirmation that the protective potential exerted by SAC begins at the early stages of excitotoxicity and neurodegeneration and supports the design of integral prophylactic/therapeutic strategies to reduce the deleterious effects observed in neurodegenerative disorders with inherent excitotoxic events.