Directed by Scott Cooper and written by Mark Mallouk and Jez Butterworth, it is based on Dick Lehr and Gerard O'Neill's 2000 book Black Mass: The True Story of an Unholy Alliance Between the FBI and the Irish Mob.

The film features an ensemble cast led by Johnny Depp as Bulger, alongside Joel Edgerton, Benedict Cumberbatch, Kevin Bacon, Jesse Plemons, Peter Sarsgaard, Dakota Johnson, and Corey Stoll.

In 1975, James "Whitey" Bulger, leader of the Winter Hill Gang, controls most organized crime within South Boston, along with his right-hand man Stephen Flemmi, newcomer Kevin Weeks, and callous hitman Johnny Martorano.

FBI agent John Connolly returns to the area, having grown up in South Boston as a friend of Whitey and his brother, Massachusetts Senate president William "Billy" Bulger.

When Connolly demands information on the Angiulos' racketeering locations, Whitey gets pictures of the rival gang's hideouts, allowing the FBI to plant wiretaps.

His wife, Marianne, sees negative changes in her husband as his agent-informant relationship with Whitey grows, including accepting expensive gifts and money from the gang.

When "bulldog" prosecutor Fred Wyshak is appointed the new assistant U.S. Attorney in Boston, Connolly attempts to make friends and perhaps divert his attention from Whitey.

Morris, disillusioned and fearing prosecution, divulges Connolly's and Whitey's relationship to The Boston Globe, and a front-page story exposes the FBI's links to organized crime.

As Connolly, Flemmi, Weeks, and Martorano are arrested, Whitey gives Billy a final goodbye from a gas station pay phone before leaving Boston.

An idea for a film adaptation of Black Mass: The True Story of an Unholy Alliance Between the FBI and the Irish Mob garnered interest intermittently since 2000.

[23] Oliver's then-newly founded Cross Creek Pictures film production company took over financing for Black Mass, and Sheridan's exit was confirmed when Russell Gewirtz was hired in 2011 to write another draft for the adaptation, and Barry Levinson was attached to direct instead.

[28] Johnny Depp's involvement with Black Mass dates back to February 2013, when he was attached to star in the film, while Barry Levinson was still slated to direct and principal photography was scheduled to begin in May 2013.

Depp briefly exited the project shortly after its sale at the 66th annual Cannes Film Festival, because of a salary dispute with Cross Creek Pictures.

[16] On June 26, Kevin Bacon was added to the cast to play Charles McGuire, the FBI Special Agent in charge of the Boston field office and John Connolly's boss.

Edgerton developed his portrayal of Connolly by studying past footage of the FBI agent before his imprisonment, some of which include his appearances on talk shows and in courtrooms.

[50] On June 9, Depp's 51st birthday, he was filming scenes on location in Quincy,[51][52] while Dakota Johnson was in Back Bay, playing Whitey Bulger's longtime former girlfriend, Lindsey Cyr.

[53] On June 11, shooting was underway in Lynn, where the crew was filming scenes in which Bulger and Stephen Flemmi pick up a prostitute named Deborah Hussey (played by Juno Temple) from the police station.

[62] A four-story building on 6 Gilson Road was transformed to resemble the Princess Eugenia apartments in Santa Monica, California, where FBI agents arrested Bulger on June 22, 2011.

[8] In the United States and Canada, the film was released alongside Maze Runner: The Scorch Trials and Captive, and was projected to earn around $26 million from 3,188 theaters in its opening weekend.

The site's critical consensus reads, "Black Mass spins a gripping yarn out of its fact-based story – and leaves audiences with one of Johnny Depp's most compelling performances in years.

[86][87][88] The Hollywood Reporter's Todd McCarthy, in addition to praising the film, called Depp's performance "fully convincing and frightening", saying it is "very welcome at this point in his career and one of his best.

"[86] Similarly, Indie Wire's Eric Kohn praised the cast and highlighted Depp's performance and Cooper's directorial efforts to depict "the haunting legacy of Bulger's criminal deeds, and the lingering sense that no justice can erase their impact.

"[89] Admiring Black Mass as an "elegantly understated crime drama", Variety's Scott Foundas positively compared Depp's performance in the film to his earlier roles with Tim Burton.

[88] BBC Culture's Nicolas Barber found some of the subplots to be unnecessary (specifically mentioning Benedict Cumberbatch as Billy Bulger), and argued that "the film is never bad, but, given the gob-smacking true story, it's disappointing that it's not great".

[92]Weeks also disagrees completely with his portrayal in the film, for instance, the suggestion the loss of his son had any impact on his behavior, or that Bulger ever once discussed business at home or with his brother.

[92] Lindsay Kimble of People believed Depp deserved an Academy Award nomination for his performance as Bulger and was surprised when his name was excluded from the list of nominees.