• Steve Buscemi as Enoch "Nucky" Thompson – the corrupt treasurer of Atlantic County and its most powerful political figure. He is often involved with various mafias and mobs out of New York and Chicago, as well as bootlegging liquor throughout Atlantic City. He is loosely based on the Atlantic City political boss Enoch L. Johnson.
• Michael Pitt as James "Jimmy" Darmody (seasons 1–2) – Nucky's onetime protégé, an honor student who left Princeton to serve in World War I. He works briefly for Nucky before striking out on his own into organized crime. He is married to Angela, and is the son of Gillian Darmody and Commodore Louis Kaestner.
• Kelly Macdonald as Margaret Thompson – a young widow and mother who turns to Nucky for help out of an abusive marriage, becoming his mistress and later his wife.
• Michael Shannon as Nelson Van Alden / George Mueller – a former straitlaced Bureau of Prohibition agent on the run. Under the alias "George Mueller", he becomes a bootlegger in the Chicago area working as the muscle for Dean O'Banion's organization and in Johnny Torrio's organization.
• Shea Whigham as Elias "Eli" Thompson – Nucky's younger brother and sheriff of Atlantic County, who works as part of Nucky's organization and later Al Capone's organization. He has numerous children, including eldest son Willie. He is based on Alf Johnson.
• Aleksa Palladino as Angela Darmody (seasons 1–2) – Jimmy's wife and the mother of his young son.
• Michael Stuhlbarg as Arnold Rothstein (seasons 1–4) – a powerful New York City gangster and multimillionaire who does business with Nucky. Charlie Luciano, Meyer Lansky, and Benjamin Siegel work for him.
• Stephen Graham as Al Capone – a violent Chicago gangster who is the right-hand man of Chicago crime boss Johnny Torrio.
• Vincent Piazza as Charles "Lucky" Luciano – a hot-headed young New York City gangster and associate of Rothstein's.
• Paz de la Huerta as Lucy Danziger (seasons 1–2) – Nucky's former mistress who gets involved with Van Alden.
• Michael Kenneth Williams as Albert "Chalky" White – a powerful African-American gangster in Atlantic City.
• Anthony Laciura as Edward Anselm "Eddie" Kessler (seasons 1–4) – Nucky's loyal butler.
• Paul Sparks as Mieczyslaw "Mickey Doyle" Kuzik – a doltish Atlantic City bootlegger who works under Nucky.
• Dabney Coleman as Commodore Louis Kaestner (seasons 1–2) – Nucky's mentor and predecessor in Atlantic City, and Jimmy's father. John Ellison Conlee portrays a young Commodore in the season-five flashbacks. The character of Kaestner is loosely based on Louis Kuehnle.
• Jack Huston as Richard Harrow (recurring season 1; regular seasons 2–4) – a former Army marksman who allies with Jimmy. Disfigured in the war, he wears a tin mask over half of his face.

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Emmy Award-winner Terence Winter, who served as executive producer and writer on the HBO series The Sopranos, was hired on June 4, 2008, to adapt the nonfiction book Boardwalk Empire: The Birth, High Times, and Corruption of Atlantic City. Winter had already been interested in creating a series set in the 1920s, feeling that it had never properly been explored before, so he decided to focus his adaptation of the book section about Prohibition. On September 1, 2009, it was announced that Academy Award-winning director Martin Scorsese would direct the pilot. It would be the first time he had directed an episode of television since an episode of Steven Spielberg's Amazing Stories in 1986. The production would be very ambitious, with some even speculating it would be too large scale for television. "I kept thinking, 'This is pointless. How can we possibly afford a boardwalk, or an empire?'" says creator Terence Winter. "We can't call it 'Boardwalk Empire' and not see a boardwalk." The production would eventually build a 300-foot-long (91 m) boardwalk in an empty lot in Brooklyn, New York, at the cost of $5 million. Despite a reported budget of up to $50 million, the pilot's final budget came in at $18 million.

On why he chose to return to television, Scorsese said, "What's happening the past nine to 10 years, particularly at HBO, is what we had hoped for in the mid-'60s with films being made for television at first. We'd hoped there would be this kind of freedom and also the ability to create another world and create long-form characters and story. That didn't happen in the 1970s, 1980s, and in the 1990s, I think. And of course... HBO is a trailblazer in this. I've been tempted over the years to be involved with them because of the nature of long-form and their development of character and plot." He went on to praise HBO by saying, "A number of the episodes, in so many of their series, they're thoughtful, intelligent [and] brilliantly put together... It's a new opportunity for storytelling. It's very different from television of the past."
"Scorsese is an actor magnet," Winter said. "Everybody wants to work with him. I had all these pictures on my wall and I thought, 'I'd really better write some good stuff for these people.'" In casting the role of Nucky Thompson (based upon real-life Atlantic City political boss Enoch L. Johnson), Winter wanted to stray from the real life Johnson as much as possible. "If we were going to cast accurately what the real Nucky looked like, we'd have cast Jim Gandolfini." The idea of casting Steve Buscemi in the lead role came about when Scorsese mentioned wanting to work with the actor, whom Winter knew well having worked with him on The Sopranos. Winter sent the script out to Buscemi, who responded very enthusiastically. "I just thought, 'Wow. I'm almost sorry I've read this, because if I don't get it, I'm going to be so sad.' My response was 'Terry, I know you're looking at other actors'... and he said, 'No, no, Steve, I said w…

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To promote the second season, which started on September 25, 2011, the producers paid the Metropolitan Transportation Authority $150,000 to decorate and operate a New York Transit Museum train of four retired New York City Subway Lo-V cars for each weekend in September 2011. The two pairs of subway cars used were 5290 & 5292 (both built in 1917) and 5443 and 5483 (both built in 1924). The train operated between noon and 6 pm, and as an express between 96th Street and Times Square–42nd Street Subway on the IRT Broadway–Seventh Avenue Line. While the MTA runs "nostalgia trains" each year, this was the first time they had been used for an advertising tie-in.

HBO also sponsored an Atlantic City beautification project with the tag line "Compliments of Nucky Thompson" and paid for eastbound tolls into Atlantic City on the AC Expressway for the weekend of September 24, 2011.