Plantar soft tissue stiffening in diabetes leads to a risk of developing ulcers. There are relatively few studies providing methods for quantifying the mechanical properties of skin and fat in the plantar tissue of diabetic patients. Previous studies used linear or non-linear single layer deformable models or linear multi-layer models. This study aimed to investigate the mechanical properties of plantar soft tissue using multi-layer, non-linear models to estimate more accurate mechanical properties in the plantar tissues of diabetic patients. Ten healthy young (HY) subjects, ten healthy old (HO) subjects, and ten old diabetic patients (DB) volunteered for the study. Indentation tests were performed at two sites in the heel. The subjects underwent computed tomography (CT) to measure the respective thicknesses of the skin and fat at the indentation sites. Subject-specific finite element models were created to estimate the parameters of the first-order Ogden forms of the skin and fat. The initial shear modulus for the fat layer μF in DB, HO, and HY were 4.68 MPa (±0.87), 2.71 MPa (±1.25), and 2.27 MPa (±0.87), respectively. The initial shear modulus for the skin layer (μS) in DB, HO, and HY were 5.86 MPa (±2.51), 7.05 MPa (±1.94), and 14.58 MPa (±1.98), respectively. The DB had stiffer fat tissue than the normal subjects in the same age group but had the same soft skin. These aspects can cause different mechanical stress conditions in a diabetic foot than in a normal foot under the same mechanical loading, making the diabetic foot vulnerable to the initiation of mechanical breakdowns such as ulcers.