Carbon (C), as a crucial trace element in nickel-based superalloys, exerts a significant influence on mechanical properties even with minor variations in content. This study investigated the effect of C content on the microstructure and mechanical properties of a novel nickel-based superalloy GH4750. The results demonstrate that with increasing C content from 0.024wt% to 0.042wt% and then to 0.082wt%, the quantity and average size of M_C carbides increase. These carbides can effectively pin grain boundaries, resulting in a progressive reduction in grain size from approximately 35 μm to approximately 31 μm and ultimately to approximately 22 μm. Both the room-temperature and 750 °C tensile strengths initially increase and then decrease as the C content increases. Conversely, the elongation displays the opposite trend. The differences in tensile properties are closely related to the carbide characteristics. An appropriate increase in carbide content can inhibit the formation and coalescence of microvoids, effectively pin grain boundaries, and prevent grain boundary slip at elevated temperatures. Excessive carbides act as preferential sites for microcrack initiation. The stress-rupture life decreases with increasing C content, attributed to the larger grain size. Based on comprehensive consideration, the optimal C content for the novel nickel-based superalloy GH4750 is determined to be 0.042wt%.