The long-tailed hermit was formally described by the Swedish naturalist Carl Linnaeus in 1766 in the twelfth edition of his Systema Naturae under the binomial name Trochilus superciliosus.

[3] Linnaeus based his description on the "colibry à longue queue de Cayenne" that had been described and illustrated in 1760 by the French zoologist Mathurin Jacques Brisson.

[10][11] During the breeding season, male long-tailed hermits sing in communal leks of up to several dozen birds, and also wiggle their long tails in display.

She lays two white eggs in a conical nest of fibres and cobwebs suspended under a large Heliconia or banana leaf.

Long-tailed hermits are trap-line feeders; they do not defend territory, but visit seasonal flowers on routes through the forest up to 1 km long.