Regeneration of orthopedic soft and hard tissues, such as ligaments, bone, and the tendon-to-bone insertion site (TBI), is problematic due to a lack of suitable biomaterials which possess appropriate mechanical properties capable of promoting cellular functions in these tissues with limited regenerative capacity. Additionally, surgically implanted biomaterials are susceptible to bacterial infection, which can lead to implant failure, as well as further complications such as wide-spread infection. To address these issues, the current study investigated magnesium oxide (MgO) nanoparticles as novel materials to improve orthopedic tissue regeneration and reduce bacterial infection. Poly (l-lactic acid) (PLLA) was mineralized with MgO nanoparticles and tested for its mechanical properties, bactericidal efficacy, and its ability to support the growth of fibroblasts and osteoblasts. These MgO nanocomposites were compared to PLLA mineralized with nanoparticles of hydroxyapatite (HA), which have been shown to promote bone tissue growth and have been widely used as materials for bone tissue engineering. Results indicated for the first time that MgO nanoparticles increased the adhesion and proliferation of osteoblasts and fibroblasts compared to plain PLLA and PLLA-HA nanocomposites. Furthermore, MgO nanocomposites showed excellent bactericidal efficacy, killing nearly all of the bacteria seeded onto them, whereas HA nanocomposites showed increased bacterial growth compared to plain PLLA. Mechanical tensile testing revealed that the addition of a secondary nano-phase to plain PLLA increased the material elastic modulus and reduced material elasticity. Moreover, the mechanical properties could be tuned to match those of bone or ligament tissue by varying nanoparticle size and concentration within the composite.