The eastern tailed blue has a wingspan of less than one inch. The hindwing has a white-tipped tail. The upperside of the male's wings is iridescent blue with a black border and white edging. There is an orange crescent above the tail. The spring generation females are dark brown on the upperside with blues scales on the inner half of the wing. The upperside of the summer generation female's wings is dark brown. The hindwing of all females has an orange crescent above each tail. The underside of the wings of both sexes is gray-white with black spots, and two, black spots near the tail each have an orange crescent.
The eastern tailed blue lives statewide in open fields, pastures, yards and forest openings. It flies close to the ground. The larvae eat legumes. The adults eat flower nectar. Adults are often seen on the ground where damp soil is present. Adults are active from April through October. The species overwinters in the larval stage.