The study aimed to compare and analyze the mechanical property and fracture behavior of LM4 composites reinforced with TiB2(1–3 wt.%) and Si3N4(1–3 wt.%) ceramic powders. A two-stage stir casting process was employed for the effective preparation of monolithic composites. To further enhance the mechanical properties of composites, a precipitation hardening treatment (both single (SSHT) and multistage (MSHT), followed by artificial aging at 100 and 200 °C) was conducted. From mechanical property tests, it was understood that in both the monolithic composites, the properties improved with an increase in wt.% of reinforcements, and composite samples subjected to MSHT + 100 °C aging treatment bested other treatments in terms of hardness and UTS values. Compared to as-cast LM4, there was a 32 and 150% increase in hardness and a 42 and 68% increase in UTS for as-cast and peak-aged (MSHT + 100 °C aging) LM4 + 3 wt.% TiB2composites, respectively. Similarly, there was a 28 and 124% increase in hardness and a 34 and 54% increase in UTS for as-cast and peak-aged (MSHT + 100 °C aging) LM4 + 3 wt.% Si3N4composites, respectively. Fracture analysis of the peak-aged composite samples confirmed the mixed mode of fracture in which brittle mode was dominating.