There is an ever increasing demand for high energy density storage devices for pulse power applications. Polymer composites offers a solution as it incorporates the high charge storage capability of the filler particles with graceful failure mechanisms of the matrix polymer. A significant challenge in realizing the potential of these systems is the poor interaction between the filler particles and the polymer. This work attempts to develop a composite with novel mixed metal phosphonate fillers. These fillers have been observed to better disperse in the polymer matrix compared to unmodified and surface-modified fillers. In addition, an embedded-sphere method was developed to measure dielectric breakdown strength in composites. This technique helped in minimizing low-field breakdown events due to the deleterious effects of the surrounding medium. A combination of these approaches has resulted in a 3-fold increase in stored electrical energy in composites over plain polymer.