Biochar is one of the new potential sustainable substitutions for ordinary Portland cement that has been studied to reduce the demand for ordinary Portland cement. This replacement can significantly cut down the environmental impacts of the construction industry and reduce the carbon dioxide emission of concrete and the industry. According to the literature review, one of the main barriers against using biochar in the construction industry is that the role and influence of biochar on the properties of cementitious composites have not been conclusively determined. Accordingly, the current study examines the role and effect of biochar on the strength and structure of cementitious mortar. To this end, the effect of using biochar as both cement replacement and additive was examined by manufacturing and testing 81 mortar cubic specimens with ultrasonic pulse velocity and compression tests. Findings indicated that, although biochar improves internal hydration by providing additional water and surface, adding biochar to the mortar reduces the compressive strength of mortar specimens. It is worth mentioning that specimens containing 15% or less biochar can completely meet the minimum strength requirement for masonry mortar based on ASTM C270. The ultrasonic results revealed that the main reason for the strength reduction of the specimens containing biochar is the low strength of biochar particles. Accordingly, some insights were proposed to enlighten the way for future studies.