Professor Frankenstein (Whit Bissell), a guest lecturer from England, talks Dr. Karlton (Robert Burton) into becoming an unwilling accomplice in his secret plan to actually assemble a human being from the parts of different cadavers. After recovering a body from a catastrophic automobile wreck, Professor Frankenstein takes the body to his laboratory/morgue, where in various drawers he keeps spare parts of human beings. The Professor also enlists the aid of Margaret (Phyllis Coates), as his secretary, to keep all callers away from the laboratory.
Margaret, becoming suspicious of what is going on, decides to investigate and goes down to the morgue. She is panic-stricken by the monster (Gary Conway), who has been activated following the grafting of a new leg and arm. She dares not tell the Professor about her feelings and keeps silent for the present. On a couple of occasions, the professor takes discarded human body parts...and feeds them to an alligator concealed in a hidden chamber.
One night, the monster leaves the laboratory. He peers into a girl's apartment. The girl becomes hysterical and starts screaming; in his attempt to silence her, he kills her in panic and flees. The next morning, the hunt for the murderer is on. Margaret, angry at the Professor, tells him that she knows that the monster is responsible for the murder. The Professor, taking no chances, has the monster kill her and feeds her remains to the alligator. Dr. Karlton, sent out of town, knows none of this.
The Professor accompanies the monster to a Lover's Lane, where he kills a teenage boy in order to obtain his face. The boy's face is successfully grafted onto the monster. Professor Frankenstein tells Dr. Karlton of his plans to dismember his creation and ship him in various boxes to England and then return there to put him together again. When they strap the monster down again, he becomes suspicious and tears loose—to throw Dr. Frankenstein into the alligator pit—while Dr. Karlton runs for help.
When Dr. Karlton arrives with the police, the monster, maddened with fright, backs into the electrical dial board. Contact with the iron wrist bands electrocutes him, and he falls to the ground, dead. Karlton tells the police that he will never forget the way the monster's face looked after the accident.
I Was a Teenage Werewolf had been a big success for AIP, and a Texas exhibitor requested two new horror movies from the studio if they could be ready by Thanksgiving. American International Pictures commissioned Herman Cohen to make I Was a Teenage Frankenstein, and Blood of Dracula. Cohen says the two films were written and put in front of the cameras in only four weeks, "so I had to really, really cut down" in terms of production values.
I Was a Teenage Frankenstein was filmed in black-and-white, with the ending in color for a vivid effect. The film was shot at Ziv studios. Cohen says that the alligator they hired for the movie had been used to dispose of bodies by serial killer Joe Ball from a small town outside San Antonio.
Upon its 1958 release, film critic Richard W. Nason, in his review for The New York Times, said,
If you discount any immediate connection between the mass media and the temper of the culture, then the film warrants little attention ... The automaton, enacted by Gary Conway, is a teenager assemble[d] from the limbs of other teenagers. This is, in one sense, abhorrent. It forces one to acknowledge the impression that such films may aggravate the mass social sickness euphemistically termed 'juvenile delinquency' ... In this particular film, there are graphic displays of human dismemberment. Before one such act of surgical perversion, the mad doctor'[s] assistant says 'I have no stomach for it.' That would be a plausible reaction for any adult who had read the day's headlines about teenage crime."
In 2009, film historian and critic Leonard Maltin dismissed I Was a Teenage Frankenstein as "campy junk ... Doesn't live up to that title, worth catching only for Bissell's immortal line, 'Answer me! You have a civil tongue in your head! I know – I sewed it in there!'".
I Was a Teenage Frankenstein was released on VHS/NTSC videocassette in 1991 by RCA/Columbia Pictures Home Video under the shortened title Teenage Frankenstein, which was the original theatrical title also used when released in the UK by Anglo-Amalgamated.
As of 2015, all rights to the motion picture I Was a Teenage Frankenstein and its derivatives were the property of Susan Hart, the widow of producer James H. Nicholson.