Motivated by a persistent need to produce a sustainable and eco-friendly construction unit, an experimental investigation was undertaken to explore the durability of a new alternative fly ash-wood fiber masonry block. The block's ability to resist different weathering actions was investigated by performing three different tests. The mechanical characterizations were investigated after exposing the specimens to eight years of harsh weather conditions such as freezing, thawing, snow, rain, high temperature, sunlight, and humidity. To test the new blocks under more rough conditions, the block's performance was investigated by exposing the specimens to 250 different environmental cycles to simulate 20 years of the typical in-situ weather conditions of the Central US through a computer-controlled environmental chamber. Another test was performed by measuring the new product resistance to rapid freezing and thawing according to ASTM C666-03. Samples from the new fly ash-wood fiber masonry block and the conventional block were completely surrounded by water in a freezing-and-thawing apparatus using Procedure A. The relative dynamic modulus of elasticity and durability factor were calculated and compared with those of the conventional masonry blocks. The results indicated that the sustainable fly ash-wood fiber masonry block was more durable and had a higher resistance to the harsh environmental conditions. The results were compared with the initial manufacturing results, the conventional masonry blocks.