| Steve Buscemi | Enoch 'Nucky' Thompson |
| Michael Pitt | James 'Jimmy' Darmody |
| Kelly Macdonald | Margaret Schroeder |
| Michael Shannon | Nelson Van Alden |
| Shea Whigham | Elias 'Eli' Thompson |
| Aleksa Palladino | Angela Darmody |
| Michael Stuhlbarg | Arnold Rothstein |
| Stephen Graham | Al Capone |
| Vincent Piazza | Lucky Luciano
(credit only) |
| Paz de la Huerta | Lucy Danziger
(credit only) |
| Michael Kenneth Williams | Chalky White
(credit only) |
| Anthony Laciura | Eddie Kessler |
| Paul Sparks | Mickey Doyle
(credit only) |
| Dabney Coleman | Commodore Louis Kaestner |
| Gretchen Mol | Gillian Darmody |
| Tom Aldredge | Ethan Thompson |
| Greg Antonacci | Johnny Torrio |
| Dana Ivey | Mrs. McGarry |
| Erik Weiner | Agent Sebso |
| Emily Meade | Pearl |
| Sean Allison | Boxer #1 |
| David Aaron Baker | Bill Fallon |
| Steven L. Barron | Reporter #1 |
| Susan Blommaert | Irene Davis |
| Pearce Bunting | Bill McCoy |
| Kenneth Cavett | Drunken Celt |
| Robert Clohessy | Alderman Jim Neary |
| John Treacy Egan | Duncan O'Connor |
| John Joseph Gallagher | Celt #1 |
| Lucy Gallina | Emily Schroeder |
| Josie Gallina | Emily Schroeder |
| William Hill | Ward Boss O'Neill |
| Austin Jones | Waiter Martin |
| Judson Jones | John #1 |
| John Keating | Ernie Moran |
| Laurence Lowry | Celt #2 |
| Raymond McAnally | Workman #1 |
| Edward McGinty | Ward Boss Boyd |
| Declan McTigue | Teddy Schroeder |
| Rory McTigue | Teddy Schroeder |
| Edelen McWilliams | Bettyanne Mulhaney |
| Brady Noon | Tommy Darmody |
| Connor Noon | Tommy Darmody |
| Nic Novicki | Carl Heely |
| Jack O'Connell | Celt #3 |
| Mikey Post | Mike Green |
| Mark Povinelli | Kevin Kiley |
| Christopher James Quinn | Reporter #2 |
| John Rue | Mayor Harry Bacharach |
| Faith Sandberg | Kitty |
| Cezary Skarzynski | Boxer #2 |
| Anna Stone | Bridget Walsh |
| Nisi Sturgis | June Thompson |
| Susan Varon | Madam Regina |
| Victor Verhaeghe | Ward Boss Fleming |
| Margot Jewers | Temperance Singer |
| Fran Lieu | Temperance Singer |
| Nicole Nelson | Temperance Singer
(as Nicole Lackey) |
| Erin Mae Miller | Temperance Singer |
| Lynne Valley | Temperance Singer |
| Nick Diamantis | Waiter |
| Martin Abrahams | Opium Den Patron
(uncredited) |
| Michael Broussard | Celt
(uncredited) |
| Todd Fredericks | Postal Worker
(uncredited) |
| Keith Leonard | Doorman
(uncredited) |
| Paul Jude Letersky | Postal Supervisor
(uncredited) |
| Mateo Moreno | Atlantic City Dandy
(uncredited) |
| Lindsay Zehra Ozgunay | Al Capone's Brothel Worker
(uncredited) |
| David Raimo | Postal Worker
(uncredited) |
| Rich Rothbell | Drunken Irishman
(uncredited) |