A study of the molecular phylogeny of bishops and widowbirds published in 2008 found that it formed part of a clade along with the Fire-fronted Bishop (E. diadematus), Black-winged Red Bishop (E. hordeaceus), Northern Red Bishop (E. franciscanus), Southern Red Bishop (E. orix), Zanzibar Red Bishop (E. nigroventris) and Red-collared Widowbird (E. ardens).
The Long-tailed Widowbird feeds on a distinct variety of seeds, including those of Setaria sphacelata (Twisted-leaf bristle grass), Paspalum dilatatum (Common paspalum), Paspalum distichum (Couch paspalum), Pennisetum clandestinum (Kikuyu grass), Triticum (wheat), Themeda triandra (Rooigras), and Senecio juniperinus (groundsel).