Brian Garfield, in an intro to a reprint of the novel on which this film is based, notes that it was originally written by director John Flynn as a period piece, intending to be set in the postwar 1940s. That's why such "film noir" veterans such as Elisha Cook Jr., Richard Jaeckel, Marie Windsor, and Jane Greer appeared in it. The studio, however, decided it would be too expensive to shoot a period picture, so the script was superficially updated, the World War II vets became Vietnam vets, and actors and actresses like Robert Ryan, Karen Black, and Sheree North joined the cast. The result was that the story was restored to its original conception (the series on which this book was based was written and set in the then contemporary 1960s and 1970s). Had the period piece idea gone through, this would have represented a rare case of backdating a character.
The character name of Parker was changed to Macklin for this film, based on Richard Stark's (Donald E. Westlake's) novel of the same name. Parker had been first introduced in Stark's "The Hunter" (1962), which had been filmed by John Boorman as Point Blank (1967). "The Outfit" was the third Richard Stark novel that featured Parker.
The filming schedule was re-arranged so Karen Black could have her wedding and honeymoon as originally planned, before she signed to do this movie.
Film critic Roger Ebert said that this was the first movie in a long time to resurrect that great piece of 1930's slang language: "gunsel".