Each flower is bilaterally symmetrical, with five 10 to 15 millimeter long petals fused into a corolla tube, and four stamens.

The fruit is a round 3 to 7 millimeter long dry capsule that splits open when ripe.

It has been recorded in the states of Alabama, Arkansas, Colorado, Connecticut, Washington, D.C., Delaware, Florida, Georgia, Iowa, Illinois, Indiana, Kansas, Kentucky, Louisiana, Massachusetts, Maryland, Maine, Michigan, Minnesota, Missouri, Mississippi, North Carolina, North Dakota, Nebraska, New Hampshire, New Jersey, New Mexico, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Virginia, Vermont, Wisconsin, West Virginia, and Wyoming, as well as the Canadian provinces of Manitoba, New Brunswick, Ontario, and Quebec.

Agalinis tenuifolia is listed as a species of special concern in the state of Rhode Island.

[8] Agalinis tenuifolia uses haustoria to connect its roots with those of its host plants, but it also has green tissues, and performs photosynthesis.