It was long considered to be a part of the superspecies for fragile ferns, as Cystopteris fragilis (L.) Bernh.

The parent diploid species are Cystopteris protrusa and the hypothesized Cystopteris hemifragilis, believed to be an extinct species.

Mackay's fragile fern grows on rock or in scree, generally in sheltered spots, in the northeastern United States.

It may be distinguished from the somewhat similar Cystopteris tennesseensis by the fact that it grows on acid substrate, while the tennesseensis grows on calcareous substrate.

The fronds of tenuis are broader, with generally alternate pinnae, while tennesseensis is narrower, with generally opposite pinnae.