The plastron is yellowish with dark lines, but there are no spots or blotches. There are two hinges present, one toward the front and one toward the back. The ground color of the skin is black or dark olive. There are no markings on the skin except that the underside of the chin, throat, and limbs tend to be lighter in color. The absence of markings, and the appearance of the shell distinguishes the mud turtle from all other Iowa turtles.
Males are slightly larger than females and have a longer, thicker tail.
Hatchling mud turtles are less than an inch. They have small yellow dots present on the edge of the carapace, and no vertebral keel. The subspecies of the yellow mud turtle, Kinosternon flavescens found in Iowa is the Illinois mud turtle, K. f. spooneri. Some authorities do not recognize this subspecies, however, and treat Iowa specimens as the nominate race. The mud turtle is only found in few, small scattered populations along the Mississippi Alluvial plain in eastern Iowa.