As related from In Search of Wonder (1956), Damon Knight wrote:
The book is full of good ideas, every other one of which is immediately dropped and kicked out of sight. The characters are child's drawings, as blank-eyed and expressionless as the author himself in his back-cover photograph. The plot limps. All the same, the story could have been an admirable minor work in the tradition of Dracula, if only the author, or somebody, had not insisted on encumbering it with the year's most childish set of 'scientific' rationalizations.
Galaxy reviewer Groff Conklin described Legend as "a weird [and] rather slow-moving first novel ... a horrid, violent, sometimes exciting but too often overdone tour de force." Anthony Boucher praised the novel, saying "Matheson has added a new variant on the Last Man theme ... and has given striking vigor to his invention by a forceful style of storytelling which derives from the best hard-boiled crime novels".
Dan Schneider from International Writers Magazine: Book Review wrote in 2005:
... despite having vampires in it, [the novel] is not a novel on vampires, nor even a horror nor sci-fi novel at all, in the deepest sense. Instead, it is perhaps the greatest novel written on human loneliness. It far surpasses Daniel Defoe’s Robinson Crusoe in that regard. Its insights into what it is to be human go far beyond genre, and is all the more surprising because, having read his short stories--which range from competent but simplistic, to having classic Twilight Zone twists (he was a major contributor to the original TV series)--there is nothing within those short stories that suggests the supreme majesty of the existential masterpiece I Am Legend was aborning.
In 2012, the Horror Writers Association gave I Am Legend the special Vampire Novel of the Century Award.