Soft piezoelectric composites (SPCs) show great promise as next-generation energy harvesting materials, with the potential to outperform piezoelectric polymers with limited elastic stretchability (e.g., PVDF) and traditional brittle piezoelectric ceramics (e.g., PZT). Presently, however, SPCs remain an emerging class of materials, with relatively few comprehensive investigations holistically exploring their synthesis, electro-mechanical property characterization, large-strain constitutive modeling, and non-linear mechanics and dynamics. In this paper, we take first steps toward addressing this compelling research opportunity. A three-component SPC is synthesized, consisting of an ultra-stretchable Ecoflex silicone rubber matrix, micron-sized PMN-PT piezoelectric particles, and CNTs that serve as inter-particle conductive “bridges.” Mechanical, electrical, and coupled electro-mechanical properties are quantified. A thermodynamically consistent, fully non-linear, finite-strain constitutive model is presented, based on a gentle adaptation of an existing transversely isotropic non-linear electro-elastic constitutive framework. Use of the particle orientation vector as an independent variable leads to two-way coupling between mechanics and electricity not present in the isotropic counterpart of this constitutive model. A prototype free energy function with the electric field as the independent variable is proposed that captures the essential physics. This free energy leads to a compact set of non-linear, finite-strain constitutive equations whose mathematical forms have direct analogue to the linear, small-strain theory of piezoelectricity.