Chris Carter named the main character Frank Black after the former lead singer of Indie/Rock band The Pixies.

The show's level of violence sparked a short-lived call for curtailment of violence on television. Although the series was cancelled long before the actual Millennium occurred, the story of Frank Black was (more or less) resolved in a 1999 episode of The X-Files entitled "Millennium."

In the opening title sequence, and in many promotional materials, the word "Millennium" was spelled with two upper-case M's (MillenniuM) The Roman numeral MM means 2000, the year which marks the turn of the Millennium.

Reportedly, the character of Lt. Bob Bletcher was written out of the series (in the episodes "Lamentation" and "Powers, Principalities, Thrones and Dominions") because of an on-set feud between Bill Smitrovich and Lance Henriksen.

Whenever Frank Black (Lance Henriksen) enters a crime scene, his jacket or coat is always buttoned up to the very top and stays that way until he leaves.

Before he fell ill, Johnny Cash was considered for the role of Frank's father, Henry Black, in the episode "Midnight of the Century". The role eventually went to someone who has had a significant influence on series creator Chris Carter's work - Darren McGavin. In essence, Carl Kolchak is Frank Black's father.

Frank's initial encounter with the Millennium Group came in the person of Peter Watts, played by Terry O'Quinn. The Group was depicted in this and other season one episodes as being an association of former law-enforcement professionals who were called in to consult on crimes by law-enforcement agencies. The Group recognised that Frank had a unique gift for profiling, as he was able to see into the mind of the perpetrator and deduce motive and psychological make-up from crime scene evidence. Frank's gift was presented on-screen as a series of rough-cut cine-film inserts shot from the perpetrator's point-of-view. When Frank began to understand the mind of the perpetrator, he also began to see the world as if through the eyes of the killer. The debate about whether Frank was actually seeing these visions or that they were a visual shorthand and storytelling device by the producers was resolved in the second season, where Frank confirmed that he sees the visions. This was later carried on with a storyline involving Jordan Black, who was also able to see visions, showing that Frank's gift had continued down the Black family line, one that he inherited from his mother.

The Millennium Group was presented as being a complex multi-facted entity in the second season. Carter had left the day-to-day production in the hands of Glen Morgan and James Wong, a writer/producer team who had previously worked on The X-Files and Space: Above and Beyond. Morgan and Wong took the underlying religious themes of the first season and made them explicit in the origin and nature of the Group. It was divided into two opposing factions, the Owls and the Roosters. The Owls believed in a secular Millennium, where Mankind could be guided through the potential disasters of 2000 and prepared for an astronomical event that was due to occur in the 2060's. The Roosters believed in the Biblical End Time foretold in the Revelation of St. John the Divine. They believed that Mankind could not avoid the destruction that was due at the dawn of the Millennium. Instead, they sought to control the End Time through the release of a modified Marburg virus to which they had an antidote that was given to selected members and their families. The Roosters' plan was implied as negotiating themselves in to a position of control and influence through the status of their members and acquisition of controlled knowledge and religious artifacts such as a piece of the True Cross and the Hand of Saint Sebastian. The motives of Frank's patron, Peter Watts, were also called into question, as it was later shown that he had received protection for his family from the viral contagion but had not helped to protect Catherine and Jordan Black.

The final season showed Frank returning to Washington and to profiling work at the FBI. This season had more control by Chris Carter and as such downplayed the religious elements of season two in order to concentrate on spree-killers and other perpetrators. This season also displayed elements of The X-Files' success such as remote viewing and the interplay of psychic and demonic forces. Frank is joined by a young, black female partner, Emma Hollis. The Millennium Group is shown at a distance as Frank is alienated from Peter Watts. The episode "Skull and Bones" depicted a mass grave in the path of a new freeway that contained the bodies of former members of the Group. Later in the season, in the episode "Seven and One", the demonic entity fans have dubbed Legion assumes the form of one of the Group's security men. The implication is that the Group have become corrupted by the very evil it was intended to fight against. Despite Frank's warnings and the evidence of her own eyes, Emma makes a commitment at a moment of personal weakness that sees her isolated from all non-Group assistance and Frank is last seen escaping from Washington having taken Jordan from school.

The series ended not with a bang but a whimper. An episode of The X-Files, entitled "Millennium", saw Lance Henriksen and Brittany Tiplady (in a cameo appearance) reprise their roles as Frank and Jordan alongside Mulder and Scully in a tale of necromancy and zombification of former Millennium Group members on the cusp of 1999/2000. Fans have panned the episode's attempt to wrap up the loose threads of the series, criticisms that Henriksen has outspokenly shared in interviews.