Monel K-500 alloy is a Ni-based alloy that is widely used in marine environments and chemical industries because of its exceptional corrosion resistance and mechanical properties. The synergetic modulation of directional solidification and thermal processing (DS-TP) technique combines DS and in situ heat treatment in a single experiment, thus eliminating the influence of environmental changes and avoiding aging effect. To investigate the effect of the DS-TP technique on the mechanical properties of the alloy, samples were prepared using DS alone and the DS-TP technique at different growth rates. Subsequently, the microstructures and mechanical properties of the samples were analyzed. In the DS experiments, an Al₂O₃ ceramic crucible (diameter: 10 mm) containing the alloy sample was heated using a graphite heater and an electromagnetic induction coil. As the sample was superheated to 200 K, it was immersed in the Ga-In-Sn liquid at a certain pulling rate. In the DS-TP experiments, the directionally solidified sample was in situ annealed at 1223 K for 1 h and then immersed into the liquid. Subsequently, the sample was subjected to an aging process at 923 K for 5 h. The microstructure of the directionally solidified Monel K-500 alloy showed columnar γ grains with pronounced <001> texture. When the growth rate decreased, the temperature gradient at the solid-liquid interface during directional solidification increased. Lower growth rates led to a coarser microstructure, lesser microsegregation of Cu, and fewer transverse grain boundaries. As the growth rate decreased from 100 μm/s to 5 μm/s, the yield strength, tensile strength, and elongation increased from 248 MPa, 421 MPa, and 63.4% to 288 MPa, 487 MPa, and 69.9%, respectively. Unlike the case of the directionally solidified alloys, the nanoscale γʹ hardening phase precipitated in the DS-TP-treated alloys and the degree of microsegregation decreased. Further, the yield strength and tensile strength increased from 368 and 640 MPa, respectively, to 386 and 686 MPa, respectively, as the growth rate decreased from 100 μm/s to 5 μm/s. The distribution of intergranular cracks in the fractography was similar to that of large-angle grain boundaries in the region closing the fracture. With the growth rate decreasing, intragranular cracks gradually appeared, and the number of intergranular cracks decreased. Moreover, grain deviation from the preferred growth orientation increased the dislocation density and decreased the elongation. Grain refinement promoted the homogeneous distribution of dislocations and caused a slight increase in the elongation. At a growth rate of 5 μm/s, the tensile strength and yield strength of the DS-TP-treated alloy increased by 34% and 41%, respectively.