This study investigates the manufacture and engineering properties of cementitious mortar incorporating unground rice husk ash (URHA) as fine aggregate. Six mixtures of mortar were produced with using URHA to substitute for crushed sand in amounts of 0%, 20%, 40%, 60%, 80%, and 100% by volume at constant water-to-powder ratio of 0.6 and volume ratio of fine aggregate to powder of 2.5. The experimental series consisted of the flowability, density, water absorption, compressive strength, flexural strength, dynamic modulus of elasticity, ultrasonic pulse velocity, and scanning electron microscopy tested under relevant standards. The fresh and dried densities of mortar with URHA significantly reduced from 5% to 29% and from 8% to 39%, respectively, compared to mortar without URHA. The higher ratio of URHA to fine aggregate led to a darker color, and higher water absorption of cementitious mortar. Replacing of 20%–40% fine aggregate volume by URHA produced mortars with comparable compressive strength at later ages. The flexural strength, dynamic modulus of elasticity, and ultrasonic pulse velocity values of mortar presented the downtrend with increase ofURHA content; however, the developing rate of these values was meaningfully promoted at later ages due to internal curing and pozzolanic action. The present study supported the technical feasibility and environmental friendliness of cementitious mortar produced with replacing the natural fine aggregate up to 100% in volume.