The performance of air lime mortars modified by the incorporation of large amounts of nanostructured colloidal silica–nanosilica, NS – (6, 10 and 20 wt.% with respect to lime) was the main objective of this work. Fresh mixture properties (water demand, setting time and plastic shrinkage), mechanical strengths up to one year and specimens’ durability after accelerated ageing conditions (climatic chamber, freeze–thaw cycles and sulfate attack by MgSO4 corrosion test) were evaluated. NS was seen to have a strong pozzolanic activity in air lime media. Although the addition of NS gave rise to an increase in volume contraction, the superficial cracking caused by the drying shrinkage was reduced. A noticeable increase in the compressive strength values was observed in the NS-bearing mortars owing to the NS filler effect and the C–S–H formation, as proved by SEM examination and MIP analysis. A honeycomb-shaped network of C–S–H phases appeared as the prevailing microstructure in mortars with large NS additions. In the face of accelerated ageing processes, the presence of NS made the mortars more durable, delaying the progress of decay.