The use of byproducts from the food industry and the investigation of substitute sources are becoming progressively significant in fulfilling the consumer demand for animal-based protein. This study aimed to investigate the nutritional value of mutton and fish livers and their future application as a source of high-added-value proteins for supplement formulation. We performed compositional analysis (moisture, ash, crude protein, crude fat), free fatty acid (FFA) analysis, and 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, and the color, peroxide value (POV), and total phenolic composition (TPC) were assessed to evaluate the nutritional value and shelf stability of mutton and fish livers. The optimized proximate and kinetics were later used to develop chicken nuggets with different percentages of mutton and fish liver added. The formulation was tested for the textural and organoleptic properties of value-added chicken nuggets that predict consumer acceptability. Comparative analysis of the variance between mutton and fish liver showed a highly significant (
P
<0.01) decrease in moisture, ash, protein, fat, DPPH, and TPC at different days and hours. The mutton liver had relatively high antioxidant potential (25.9% DPPH and 154-mg GAE/100 g TPC) compared with the fish liver. However, the fish liver’s FFA and POV (2.4% for both) were higher than those of the mutton liver. The results showed that, after formulation, an increase in the amount of liver led to a highly significant (
P
<0.01) rise in the nutritional value of the nuggets, including a 1.5%∼2.0% increase in protein content. This research indicates that valuing mutton and fish liver as a protein replacer in processed foods can be useful in developing healthy food products.