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Although the onscreen credits include a 1971 copyright statement for L.T.M. Films, Ltd., the film was not registered for copyright. The closing cast credits feature a shot of each main character with the actor's name superimposed over the image. Onscreen acknowledgments include The Elgin Cinema and Harley-Davidson. Although George Norris is credited in the Hollywood Reporter production charts as film editor, only Angelo Ross receives onscreen credit.
At the time of the film's production, Cannon Films was owned by Dennis Friedland and Christopher C. Dewey. Although David Gils was originally named as the producer of Who Killed Mary Whats'ername, on February 23, 1971 Hollywood Reporter announced that George Manasse had replaced him. As noted in many contemporary sources, the film was shot entirely on location in New York City. According to Filmfacts, Who Killed Mary Whats'ername cost $425,000 to produce. The film marked the first time that actor Earl Hindman (1942-2003) was billed under that name. Previously he had acted under the name Leo Heinz. Hindman went on to portray the unseen neighbor, "Wilson Wilson," on the popular ABC television sitcom Home Improvement, which aired from 1991-1999. Music director Gary McFarland died on November 2, 1971, a few days before the film's release. Although most critics disliked the film, the cast was roundly praised.