The objective of this research is to evaluate microwave sintering as a viable technique to reliably process high-strength mullite and mullite–zirconia composites. In this report, the advantage of microwave processing over conventional heating is realized by comparing the densification behavior and mechanical properties of sintered mullite and mullite–zirconia composites. Commercially procured mullite powders were sintered using a 3 kW, 2.45 GHz microwave furnace in the range of 1400°–1500°C, for 1 h. For pure mullite, an increase in sintering temperature from 1400° to 1500°C decreased the porosity from 30% to 13%, which increased the compressive strength from 128±18 to 387±21 MPa with 1 wt% MgO as a sintering aid to mullite. Furthermore, yttria-stabilized tetragonal zirconia (3Y-TZP) was incorporated (up to 20 wt%) to improve the microwave absorption efficiency of mullite. A maximum density of 92% can be achieved for mullite with 1 wt% MgO and 10 wt% ZrO2 composites sintered at 1500°C in microwave furnace and exhibited a maximum hardness of ∼10.24±0.61 GPa, compressive strength of ∼740±38 MPa and a moderately high indentation fracture toughness of ∼3.65±0.43 MPa·m1/2. Microwave sintering data, when compared with those of conventional sintering, revealed that volumetric heating of microwaves led to considerable improvement in strength of mullite and mullite–zirconia composites while utilizing reduced time and energy.