Auxetic architected materials have been at the forefront of developing materials with a wide range of negative Poisson’s ratios, tunable stiffnesses, and high ductilities due to their novel deformation under uniaxial compression. Despite that, the adoption of auxetic materials in load-bearing applications has been challenged by the requirement that their bulk modulus is significantly less than that of the fully dense parent material. In this paper, we study whether using the mechanism that provides a negative Poisson’s ratio can be mimicked in an interpenetrating phase composite to enhance its matrix’s peak strength and mechanical behavior. In this case, a brittle matrix can be enhanced with a small volumetric fraction of an auxetic truss lattice. The auxetic phase behaves as reinforcement, increasing the hydrostatic compression and confinement in the matrix caused by the externally applied load and bridging matrix cracking. We study prototyping composites using a concrete/mortar matrix with 15-5PH stainless steel auxetic truss lattice reinforcement. Two families of re-entrant auxetic truss lattices were manufactured using laser powder bed fusion to explore the effects of enhancing the confinement pressure to the composite mortar/steel matrices. The results of the experimental program with LPBF-manufactured truss lattices tested under axial compression embedded in mortar composites are presented and discussed. Analytical and numerical modeling at different characteristic angles is used to decompose the effects of stiffness and Poisson’s ratio on the confining pressure generated by the reinforcing phase. A comparison with conventional reinforcing schemes with similar volumetric steel amounts reveals non-rule-of-mixtures behavior in the composite.