In this paper, the role of masonry joints in the textile reinforced mortar (TRM)-to-substrate bond is investigated through an experimental program employing double-lap/double-prism (DL/DP) shear bond tests. The TRM system is composed of a dry glass fiber textile and a cementitious mortar applied as an overlay onto unreinforced wall prisms made from solid clay bricks and a cement/lime based masonry mortar. Specimens with different bond lengths (BLs) and various mortar joints-to-bonded area ratios are examined. For all specimens, the shear bond strength of the TRM/masonry interface is larger than the respective strength of the textile/matrix interface the shear failure of which was the governing failure mechanism. For the BLs considered, an increase in the mortar joints/BL surface ratio led to an increase in the transferable shear load of the joints indicating a substrate dependent behavior. A critical discussion on the characteristics of this dependence is included in this paper highlighting the effect of the substrate on the modification of the in situ properties of the composite’s matrix. Finally, for BLs larger than the effective BL an existing indirect bond-slip model was applied, therefore, a cohesive material law could be determined for each substrate modified textile/matrix combination.