Independently made by several producers, including Edgar Lansbury, He Knows You're Alone was shot on location in Mastroianni's native Staten Island, New York in December 1979 under the working title Blood Wedding.

Although the film received mixed reviews, it was a commercial success for MGM, grossing nearly $5 million at the U.S. box office.

He Knows You're Alone has been credited for being one of the first horror films inspired by the success of Halloween, and shares a number of similarities with that previous hit.

[9] A young bride is murdered on her wedding day by the man she rejected for her current fiancé, Len Gamble, a detective.

Several years later on Long Island, a young bride-to-be named Marie is stabbed to death in a movie theater while her friend Ruthie sits beside her.

[10] Whereas other contemporaneous slasher films, such as Friday the 13th, utilize the summer camp setting as an organizing principle and locale, He Knows You're Alone takes place in various wedding-specific locations, such as a dressmaker's shop, a church, and the bride's home.

"[9] The concept for He Knows You're Alone was developed in 1979, after director Armand Mastroianni pitched an idea to producer Edgar Lansbury for a horror film based on the urban legend of "The Hook", in which a young couple in a parked car are attacked by a murderer.

[13] Caitlin O'Heaney auditioned for the role of Amy Jensen, and despite disliking horror films, agreed to take the part in order to gain entry into the Screen Actors Guild.

[21] Executive producer Joseph Beruh sold the film to the studio after taking it to Los Angeles and screening it for potential distributors, once of which was 20th Century-Fox.

[22] To promote the film, MGM devised a theatrical trailer that featured footage of actress Caitlin O'Heaney applying makeup in front of a mirror, during which a hand breaks through the glass and grabs her.

[27] On May 18, 2021, Scream Factory released the film on Blu-ray with a new 2K scan from the original interpositive, along with several new interviews with cast and crew members.

[26] The Boston Globe's Michael Blowen faulted the film's script and direction as "slow and strictly second rate", adding "the production values are only slightly better than those in my uncle's home movies".

[30] Kevin Thomas of the Los Angeles Times deemed the film a "standard grisly rampaging killer fare...  there are the usual bows to Hitchcock...  but He Knows You're Alone is really no more than just another by-the-numbers piece of sickening trash".

There are so many cinematic shock tactics employed—tacky eerie music announcing the killer's presence, shadowy forms in the foreground and background, slamming doors, blown light fuses, hands on shoulders etc.—that you're numb by the sixth killing.

"[32] Jimmy Summers of BoxOffice magazine gave the film a negative review, noting, "He Knows You're Alone is another one of those low-budget thrillers that should carry in the credits line: "Based on characters and ideas developed by John Carpenter.

"[33] Additionally, Summers noted the lack of on-screen violence as leaving the "more blood-thirsty horror fans feeling cheated".

[33] John Dodd of the Edmonton Journal similarly deemed the film "unoriginal and unnecessary", and a "bloody, boring walk down the aisle".

[34] John Herzfeld of The Courier-Journal, however, praised the film's opening film-within-a-film sequence as a "wry twist", concluding, "Despite the incompetent script and some irregular pacing, He Knows You're Alone does deliver a few surprises and some suspense".

[7] Nathaniel Thompson of Turner Classic Movies noted a favorable retrospective assessment of the film, writing, "time has proven the [slasher] subgenre to have an enduring appeal that's easily survived the slings and arrows of its attackers, with this one holding a particular fascination as an early and quirky offering at the dawn of the big studio slasher boom.