This native perennial plant is 1-3' tall, branching frequently to create a bushy effect. The slender stems are hairless. The slender opposite leaves are up to 3' long and ¼' across. Each leaf is sessile, linear, and hairless, with a prominent central vein and smooth margins. The upper stems terminate in small flat heads of flowers. The short tubular flowers are white, often with scattered purple dots, and individually about ¼' long. The corolla is divided into an upper lip and a lower lip with three lobes. The reproductive structures of each flower are white, except that the anthers are purple. The calyx is divided into several slender green lobes. The blooming period is early to mid-summer, and lasts about 11½ months. There is no floral scent, although the foliage has a mild mint scent and somewhat stronger minty taste. The small dark seeds are without tufts of hairs, but are small enough to be dispersed by gusts of wind. The root system consists of a taproot and rhizomes. Slender Mountain Mint can spread vegetatively, forming colonies of closely bunched plants.