Resistance to chloride penetration plays a crucial role in preventing premature corrosion of reinforced concrete in marine environments or when using de-icing salts. For reinforced concrete with ordinary Portland cement (OPC), the curing of concrete is vital in ensuring that the designed chloride penetration resistance of concrete is achieved. The method and duration of curing are especially important with alkali-activated slag (AAS) to allow proper reaction and development of the pore structure. AAS generally needs to be cured under closed conditions for at least 28 d to achieve the desired quality. The objective of this study was to analyse the effect of curing time on the development of chloride diffusion resistance and pore structure of alkali-activated slag mortar. Chloride diffusion resistance was analysed at 7, 28, and 90 d using non-steady-state chloride migration according to NT BUILD 492 and accelerated chloride testing according to NT BUILD 443, both of which were originally developed for OPC. Mercury intrusion porosimetry (MIP) was used to evaluate the effect of pore structure on chloride penetration. The diffusion results of alkali-activated mortar showed that high resistance to chlorides can be achieved after only 7 d of curing, which is attributed to the development of fine porosity of alkali-activated slag at an early age.