This study investigates the workability behavior of a novel non-hydraulic lime-based mortar enhanced with nano-silica (NS) and nano-alumina (NA), developed for sustainable retrofitting of historical masonry. A matrix of 20 different mortar formulations combining varying NS (0–3%) and NA (0–2%) dosages was designed. To isolate and observe solely the effects of nano-additives on final mechanical properties of the novel mortar, workability, which can have a direct effect of the final characteristics of the mortar, was designed and kept fixed with the flow diameter range of 17.5–18.5 cm for all 92 flow table tests to be excluded from the final effects. In this regard, results showed that NS was the dominant factor increasing water demand, with an almost linear and cumulative effect, while NA acted as a synergistic amplifier at higher NS contents, leading to nonlinear water demand behavior. These interactions are attributed to changes in particle packing, nucleation phenomena, and hydration film thickness. The findings highlight the critical importance of precise water control and additive balance when formulating nano-enhanced lime mortars, contributing to the design of nano-based mortar in a complete controlled method.