Njáls saga, like the other sagas of Icelanders, is anonymous. There are, however, many theories about the saga's authorship. The oldest idea, attested in the early 17th century, is that Sæmundr fróði wrote the work. Other suggested authors include Sæmundr's sons, Jón Loftsson, Snorri Sturluson, Einarr Gilsson, Brandr Jónsson and Þorvarðr Þórarinsson.
The saga is now believed to have been composed in the period from 1270 to 1290. Among written sources which the author likely used are Laxdœla saga, Eyrbyggja saga and Ljósvetninga saga as well as the lost sagas Brjáns saga and Gauks saga Trandilssonar. The author must, however, have derived the bulk of the material in the saga from oral tradition which he manipulated for his own artistic purposes. Opinions on the historicity of the saga have varied greatly, ranging from pure fiction to nearly verbatim truth to any number of nuanced views. It can be regarded as certain that Njáll and Gunnarr were real historical people and their fateful deaths are referred to in other sources. Gabriel Turville-Petre said, "It was not the author's purpose to write a work of history, but rather to use a historical subject for an epic in prose".