A wireless, passive strain sensor based on the shift of higher-order harmonic signals of a magnetically soft material is described. The strain sensor consisted of a magnetically soft element, placed over a permanent magnetic element and separated by a deformable layer. As compressive forces were exerted on the strain sensor, the dimension of the deformable layer varied, changing the separation distance between the soft and permanent magnetic elements. This in turn altered the higher-order harmonic field of the magnetically soft element, allowing remote measurement of stress and strain. In the current study, three different types of deformable layers with distinctive material properties were separately incorporated into the sensor. External forces were gradually applied on each sensor and the variations in harmonic signals were measured. The shifts in the magnetic harmonic spectrum of the sensors were linearly correlated with the mechanical alteration. Good stability, linearity and repeatability of the strain sensor were also demonstrated. This passive and wireless sensor is useful for long-term detection of mechanical loading from within an object such as inside a concrete structure or a human body.