This paper presents a focused comparative case study considering the influence of natural and synthetic fibers on the fresh and mechanical properties of concrete. Locally sourced 19 mm long sisal fibers from sisalana leaves and manufactured polypropylene fibers were incorporated in a normal strength concrete matrix with fiber volumetric contents of 1%. After describing the measured aggregate characteristics, mix designs, and fresh concrete properties, several destructive and non-destructive tests on hardened concrete were undertaken. The former included compression tests on cylinders and flexural tests on prismatic samples, and the latter included ultrasonic pulse velocity and rebound number tests. The workability of sisal-fiber reinforced concrete was generally lower than the nominal concrete and that provided with polypropylene fibers by about 20%, largely due to the hydrophilic nature of the natural fibers. Test results showed that the presence of sisal fibers can improve the compressive strength by about 6%, and the tensile strength by about 4%, compared with the non-reinforced counterpart. This was due to the sisal fibers storing moisture that was released gradually during hydration, helping with the strength development. The concrete with polypropylene had virtually identical properties to the reference concrete. In addition to fresh and mechanical properties, environmental impacts associated with the production of fiber and concrete were also identified and discussed. Based on the assessments from this paper, overall, from the two fibers investigated, the sisal fiber showed more promising results, indicating that natural fibers can be a more sustainable alternative to plastic fibers, providing a good balance between workability and strengths.