| Steve Buscemi | Enoch 'Nucky' Thompson |
| Kelly Macdonald | Margaret Thompson /
Mrs. Rohan |
| Michael Shannon | Nelson Van Alden
(credit only) |
| Shea Whigham | Elias 'Eli' Thompson |
| Michael Stuhlbarg | Arnold Rothstein |
| Stephen Graham | Al Capone |
| Vincent Piazza | Lucky Luciano |
| Michael Kenneth Williams | Chalky White |
| Anthony Laciura | Eddie Kessler
(credit only) |
| Paul Sparks | Mickey Doyle
(credit only) |
| Jack Huston | Richard Harrow
(credit only) |
| Ron Livingston | Roy Phillips |
| Jeffrey Wright | Valentin Narcisse |
| Gretchen Mol | Gillian Darmody |
| James Cromwell | Andrew W. Mellon |
| Stephen Root | Gaston Means |
| Julianne Nicholson | Esther Randolph |
| Brian Geraghty | Agent Warren Knox /
Jim Tolliver |
| Ben Rosenfield | Willie Thompson |
| Domenick Lombardozzi | Ralph Capone |
| Erik LaRay Harvey | Dunn Purnsley |
| Chris Caldovino | Tonino Sandrelli |
| Greg Antonacci | Johnny Torrio |
| Ivo Nandi | Joe Masseria |
| Arron Shiver | Dean O'Banion |
| Eric Ladin | J. Edgar Hoover |
| Patch Darragh | Robert Bennett III |
| Glenn Fleshler | George Remus |
| Dionne Audain | Mrs. Tremaine |
| Rhonda Ayers | |
| Steve Beauchamp | Sid |
| Margot Bingham | Daughter Maitland |
| Surya Botofasina | Mr. Coates |
| Joe Caniano | Jake Guzik |
| Bo Dietl | North Side Resident |
| Sean Dugan | Jeffrey Romsey |
| Kayla Ferguson | Doris |
| Renee Flemings | Mrs. Bridge |
| Emma Holzer | Edith Thompson |
| Richmond Hoxie | Harlan Stone |
| Will Janowitz | Hymie Weiss |
| Gaetano LoGiudice | |
| Marcus Naylor | |
| Paul Pilcz | |
| Joseph Riccobene | Frankie Yale |
| Sarah Shankman | |
| Franklin Ojeda Smith | Deacon Lemuel Cuffy |
| Nisi Sturgis | June Thompson |
| Robert Verlaque | Professor
(as Robert L. Verlaque) |
| Casey Webb | |
| Brandon Zumsteg | Brian Thompson |
| Chloe Arnold | Onyx Club Dancer 1 |
| Maud Arnold | Onyx Club Dancer 2 |
| Brittany Engel-Adams | Onyx Club Dancer 3 |
| Karida Griffith | Onyx Club Dancer 4 |
| Jennifer Jones | Onyx Club Dancer 5 |
| Marla McReynolds | Onyx Club Dancer 6 |
| Erin Moore | Onyx Club Dancer 7 |
| Jae Ponder | Onyx Club Dancer 8 |
| Celestine Rae | Onyx Club Dancer 9 |
| Jennifer Rias | Onyx Club Dancer 10 |
| Campbell Dunsmore | Miss Finch |
| Jared Johnston | Federal Agent |
| Giuseppe Ardizzone | Torrio Bodyguard
(uncredited) |
| Stephanie Corbett | Al Capone's Prostitute
(uncredited) |
| Chris J. Cullen | Arresting Officer
(uncredited) |
| Casey Davis | Onyx Club Member
(uncredited) |
| Joe Fionda | FBI Agent
(uncredited) |
| Heather Gilbert | Prostitute
(uncredited) |
| Brian Kealty | Federal Agent
(uncredited) |
| Charles Marleau | Onyx Club Couple
(uncredited) |
| Frank L. Messina | Frankie
(uncredited) |
| Nate Steinwachs | Federal Agent
(uncredited) |
| Gerard Sullivan | Nucky's Bodyguard
(uncredited) |