Critical reception
Upon its theatrical release, Maniac received numerous unfavorable reviews, with many critics lambasting the film for its depiction of violence. Gene Siskel did a TV piece for the CBS affiliate in New York City where he strongly criticized the use of TV kiosks that showed graphically violent and gory scenes from the film outside a theatre in Times Square; he later picked the movie as one of his "Dogs of the Week" on his show with Roger Ebert, stating that he walked out of the movie after 20 minutes because he felt it could not redeem itself at that point.
Vincent Canby of The New York Times wrote: "Good sense, if not heaven, should protect anyone who thinks he likes horror films from wasting a price of admission on Maniac, a movie that shows how an aging, pot-bellied maniac slices up young women of no great intelligence".
On the review aggregator website Rotten Tomatoes, Maniac holds a 40% approval rating based on 20 critic reviews, with an average rating of 5.2/10.
Stuart Galbraith IV (DVD Talk) said of the film: "Despite some good direction and a sincere, even daring performance by character actor Joe Spinell (Rocky), who also co-produced and co-wrote its screenplay, Maniac (1980) is alternately repellent and boring, despite the obvious intelligence that went into its making. A low-budget slasher film notable for its extremely graphic splatter effects by Tom Savini - who also appears in the picture - Maniac is mostly a character study, anticipating the much superior (if no less unpleasant) Henry: Portrait of a Serial Killer (1986)".
Tom Becker of DVD Verdict said "that the film is so effective is due in no small part to the performance of Joe Spinell as Frank, the schlubby-looking guy whose darkness overwhelms him. This is not the standard, amateurish, paint-by-numbers horror villain turn. Spinell creates a fully formed portrait of this monster that goes far beyond the surface. He mutters to himself, talks to mannequins, growls like an animal when stalking his prey—yet he can be charming as well, and while the pairing of Spinell and Munro as lovers has a definite Beauty and the Beast quality to it, it's not entirely unbelievable. Had Maniac been more of a mainstream film, Spinell might have been remembered as one of the great horror heavies".
The Hollywood Reporter cites the film as "something of a grubby touchstone among genre fans".
Film scholar John Kenneth Muir wrote that the film "positively oozes sleaze and despair, and that's a compliment...After watching Maniac, you'll want to take a deep breath, maybe even a shower, but you won't have wasted ninety minutes on something that has no meaning, no pulse, and no heart". Jim Harper, in his book Legacy of Blood: A Comprehensive Guide to Slasher Movies, praised Spinnell's performance, noting it as the "centerpiece" of the film.