Freestanding, strip-shaped magnetoelastic (ME) biosensors are a class of wireless, mass-based biosensors that are being developed for the real-time detection of pathogenic bacteria for food safety and bio-security. The mass sensitivity of these biosensors operating in longitudinal-vibration modes is known to be largely dependent on the position of masses attached to the sensor surfaces. Hence, considering this dependence is crucial to the detection of low-concentration target pathogens, including single pathogenic bacteria, because their local attachment may cause varying sensor responses. In a worst case scenario, the resultant sensor responses (i.e., mass-induced resonance frequency changes of the sensor) may be too small to be detected despite the attachment of the target pathogenic masses. To address the issue, phage-coated ME biosensors (magnetostrictive strips (4 mm × 0.8 mm × 30 μm) coated with a phage probe specifically binding streptavidin protein) with localized masses (streptavidin-coated polystyrene beads) were fabricated, and mass-position-dependence of the sensor’s sensitivity under the fundamental-mode vibration was experimentally measured. In addition, three-dimensional finite element (FE) modal analysis was performed using the CalculiX software to simulate the phenomena. The experimental and theoretical results show close agreement: (1) the mass sensitivity was low when the mass was positioned in the middle of the sensor’s longest dimension and (2) a much higher mass sensitivity was, by contrast, obtained for the equivalent masses placed at both ends of the strip-shaped sensor. Furthermore, FE models were constructed for differently sized, phage-coated ME biosensors (100 – 500 μm in length with different widths and thicknesses) loaded with a single bacterial mass (2 μm × 0.4 μm × 0.4 μm, 1.05 g/cm3) at varying longitudinal positions. The mass sensitivity was found to be approximated by a mass-position-dependent Boltzmann function whose amplitude is inversely proportional to the length squared, width, and thickness of the sensor.