Auxetic architected materials have been at the forefront of developing materials with a wide range of negative Poisson’s ratios, tunable stiffness, and high ductility 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 same 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. Our work is focused on rapidly prototyping composites using a concrete/mortar matrix with 15-5 PH stainless steel auxetic truss lattice reinforcement. The families of re-entrant bowtie and double pyramid truss lattices were manufactured using laser powder bed fusion to study the effects of increasing the confinement pressure when embedded in 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 modeling is used to decompose the effects of stiffness and Poisson’s ratio on the confining pressure generated by the reinforcing phase. Numerical results on a perfectly bonded periodic unit cell of the composite material are illustrated, presenting the auxetic confinement pressures for different characteristic angles of the architectures with a maximum increase in confining pressure of 34.4%. Our findings reveal significant enhancements in ductility and peak strength using the proposed scheme, with gains reaching up to 240% and 165%, respectively, when compared to conventionally confined specimens and a non-rule-of-mixtures behavior in the composite.