The view towards a sustainable bioeconomy is increasing the interest of using renewable natural resources in the production of composites. Until now, the production of sustainable composites has been mainly examined from the point of view of material composition and structure, by replacing petroleum-based components with those that are obtained from renewable resources known as natural fiber composites (NFCs). The usefulness of newly acquired materials is mostly evaluated considering their performance and economic costs, whereas the aspect of environmental protection is underestimated. The impact of composites that are made from renewable resources is examined within the two parts of this study—the first part compares different nitrogen (N) fertilization scenarios for plant origin (hemp and flax) fibers. When compared, hemp crops show higher CO2 accumulation, (−1.57 kg CO2 eq) than flax (−1.27 kg CO2 eq). In addition, the environmental impact of both fiber types is compared to polyamide composites, one of the traditionally used materials in the automotive industry in the second part of this study. According to the conducted life cycle assessment, Flax/PLA emits 1.19 kg CO2 eq per 1 kg composite, Hemp/PLA 1.7 kg CO2 eq per 1 kg composite, and PA66/GF 9.14 kg CO2 eq per 1 kg composite. After the comparison, it was concluded that bio-based composites are able to ensure lower CO2 emissions, because CO2 is accumulated and stored in the fibers, however the traditionally used composites are able to provide a lower impact in other environmental categories.