"A. brownii" was first discovered during the Tanager Expedition in 1923 by botanist Edward Leonard Caum. It differed from other Hawaiian species of "Amaranthus" with its spineless leaf axils, linear leaves, and indehiscent fruits. It was one of 26 vascular plants on Nihoa, 17 of which are indigenous, six alien, and three endemic only to Nihoa; these latter three included "A. brownii", the Nihoa fan palm or "loulu", and the Nihoa carnation. "A. brownii" was considered the rarest plant on Nihoa and was not directly observed on the island after 1983. Past expeditions collected plant samples and seeds, but no specimens managed to survive ex-situ conservation efforts outside of its native habitat. Consequently, there are no known plants or seeds from "A. brownii" in any botanical garden.