Morphotropic phase boundary (MPB) compositions separating rhombohedral and tetragonal phases in the (1−x−y)Pb(Mg1/3Ta2/3)O3–yPbZrO3–xPbTiO3 (PMT–PZ–PT100x) ternary solid solution system were characterized using X-ray diffraction and dielectric, piezoelectric properties. This work focused on compositions with a PZ content fixed at y=0.2, with an MPB composition found to be located at x=0.4. Piezoelectric coefficients and dielectric permittivity were found to be on the order of d33=580 pC/N and 4100, respectively. Acceptor modification using manganese was found to induce a “hardening” effect in 0.4PMT–0.2PZ–0.4PT, with decreased piezoelectric coefficients d33 and dielectric loss and increased mechanical quality factor Q. Piezoelectric coefficients d33, Q values, and dielectric loss were found to be 500 pC/N, 2000, and 0.4%, respectively, for 0.4PMT–0.2PZ–0.4PT with MnO2 dopant levels around 0.5 wt%. The figure of merit (product of Q and d33) was found to be on the order of 1 × 106, significantly higher when compared with other hard piezoelectric PZT materials. Specifically, the PMT–PZ–PT materials may be attractive candidates for high-power ultrasonic applications, particularly fine-scale components that require relating high permittivities.