We present the first experimental demonstration of two-photon Mueller matrix second-harmonic generation (SHG) microscopy for the extraction of representative Mueller matrices. Individual SHG images of porcine sample sets are divided into subimages, and the local nonlinear two-photon Mueller matrices () and degree of polarization () are determined. Results of the two-photon Mueller matrices from different sample types and the effects of thickness on the mean distribution per polarization basis are investigated. We find that the distribution shape changes and an associated bimodal mean difference () increases with thickness. Our approach has potential as a quantitative imaging technique and can be applied to other multiphoton coherent imaging modalities.