Nowadays, more and more municipal waste is being generated. One of the reasons for this is the growing number of products purchased and thus the production of municipal waste. The growing amount of waste causes problems with its processing. One way of disposing of waste is its thermal conversion in municipal waste incinerators. This process produces by-products such as ash from municipal waste incineration. The management of these products is problematic due to their toxicity. One way of managing them is to add these by-products to cement mortars and concretes. The ash used in this study was obtained from the incineration of municipal waste in Poznań. Cement mortars were made using CEM I, quartz sand, and ash from municipal waste incineration. Ash was added to the mortar in amounts of 5, 10, 15, 20, and 25% of the cement weight. Samples with appropriate amounts of ash and reference samples were prepared. The basic properties of the mortar, such as density, consistency, flexural and compressive strength after 28 days, and water absorption, were evaluated. The analysis showed that an increase in compressive strength was observed for mortars containing 5, 10, 15, and 20% ash by weight of cement. In the case of flexural strength, an increase was observed only for the 5% ash content. The results for the density of hardened mortar are similar to those for compressive strength. In summary, adding waste incineration ash to municipal waste can have a positive effect on the strength properties of mortars. However, research should be expanded to include an analysis of the toxicity of mortars and an examination of their chemical composition (heavy metal content and oxide composition).