In this study, three different spheroidal graphite cast irons with varying Mn, Mo, and Ni contents were produced. The cast irons were subjected to austenitization at 900 °C for one hour, followed by austempering at 300 °C, 320 °C, and 340 °C for 90 min. A comparison was made between the microstructure and mechanical properties of the as-cast and austempered spheroidal graphite cast irons. Microstructural examinations were carried out using optical microscopy, EBSD (electron backscatter diffraction), and XRD (X-ray diffraction) techniques. In order to characterize the mechanical properties, Brinell hardness test and tensile test were conducted. Microstructural analysis of all spheroidal graphite cast irons revealed the presence of ferrite, pearlite, and graphite phases. After austempering, acicular ferrite and retained austenite phases were observed in the microstructure, with their amounts varying according to the austempering temperature. The amount of retained austenite increased with rising austempering temperature, and the highest increase was observed in the cast iron containing highest Ni, Mo, and Mn contents. The highest hardness, yield strength, and tensile strength were also obtained in this composition. Austempering heat treatment significantly improved the mechanical properties; however, further increase in austempering temperature resulted in a decline in these properties.