UV-light-assisted additive manufacturing (AM) technologies require bio-based resins that can compete with commercial petroleum-based ones to enable a more sustainable future. This research proposes a significantly improved vegetable oil-based resin reinforced with nanofibrillated cellulose (NFC). The incorporation of ultra-low concentrations (0.1–0.5 wt%) of NFC produced disproportionate enhancements in mechanical performance. Noteworthy, a 2.3-fold increase in strain at the break and a 1.5-fold increase in impact strength were observed with only 0.1 wt% of NFC, while at 0.5 wt%, a 2.7-fold increase in tensile modulus and a 6.2-fold increase in toughness were measured. This is in spite of NFC agglomeration at even the lowest loadings, as observed via examination of fracture surfaces and dynamic mechanical analysis (DMA) Cole–Cole plot analysis. The addition of 0.1 wt% NFC also increased creep resistance by 32% and reduced residual strain by 34% following creep recovery. The Burgers model satisfactorily described the composites’ viscoelastic–viscoplastic behavior within the applied stress levels of 1–3 MPa. The successful development of novel NFC/bio-resin composites with enhanced mechanical performance and long-term stability highlights the potential of these composites to substitute petroleum-based resins in the context of AM resins.