Owing to its excellent nutritional value, eggs are among the most important components of the human diet. Gender and environmental factors, such as feed composition, may alter the nutritional profile and quality of eggs. Feed additives have recently been used to enhance the health and productivity of hens, which has resulted in the production of higher-quality eggs. The fungus Cordyceps militaris, a well-established source of traditional medicines, contains potential bioactive metabolites, which prompted us to examine the effects of C. militaris-supplemented diets on the quality of hens’ eggs. The hens of two species (Gallus gallus domesticus and Araucana) were fed with one of three different diets: a control diet and diets supplemented with 2% or 5% of C. militaris. Egg quality was determined by measuring the Haugh Unit, yolk color, and shell thickness. In addition, egg and shell densities together with the ratio of yolk to albumen were calculated. Eggshell thickness and yolk color were both enhanced by the addition of C. militaris, whereas Haugh Unit values were somewhat reduced. Egg size, eggshell weight, and yolk and albumen production were all enhanced by C. militaris supplementation. Notably, in hens fed the 2% C. militaris-supplemented diet, enhancement was more evident in the yolk than in the albumen. The overall quality of the egg yolk was enhanced when 2% C. militaris was added to the hens' diet, which led to increases in both yolk color and quantity. Eggshell thickness and weight were also higher among eggs laid by hens fed the supplemented diets. Although these effects differed depending on the chicken species, we established that, in general, C. militaris contributes to improving egg quality.