| 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 |
| Paz de la Huerta | Lucy Danziger |
| Michael Kenneth Williams | Chalky White
(credit only) |
| Anthony Laciura | Eddie Kessler |
| Paul Sparks | Mickey Doyle |
| Dabney Coleman | Commodore Louis Kaestner |
| Gretchen Mol | Gillian Darmody |
| Greg Antonacci | Johnny Torrio |
| Danny Burstein | Lolly Steinman |
| Frank Crudele | Big Jim Colosimo |
| Peter McRobbie | Supervisor Elliot |
| Erik Weiner | Agent Sebso |
| Brent Bateman | Eddie Corrigan, Chicago Tribune Reporter |
| Ellie Braverman | |
| Bill Bria | |
| Clem Cheung | Chinese Man |
| Peter Claymore | Grey-Haired Man |
| Robert Clohessy | Alderman Jim Neary |
| Bob Engel | Abe Klein |
| Lauren Champlin | Female Patient |
| Megan Ferguson | Claudia |
| Ken Forman | Stage Manager
(as Ken Foreman) |
| Lucy Gallina | Emily Schroeder |
| Josie Gallina | Emily Schroeder |
| Jordan Gelber | Simon |
| Allison Guinn | Capone's Whore |
| Sam Guncler | Croupier |
| Johnnie Mae | Louanne |
| Edward McGinty | Ward Boss Boyd |
| Declan McTigue | Teddy Schroeder |
| Rory McTigue | Teddy Schroeder |
| Connor Noon | Tommy Darmody |
| Brady Noon | Tommy Darmody |
| Nic Novicki | Carl Heely
(as Nick Novicki) |
| Katelyn O'Friel | |
| Annie Paul | Showgirl #1 |
| Joseph Riccobene | Frankie Yale |
| Allen Lewis Rickman | George Baxter |
| Kendal Ridgeway | Nurse
(as Kendel Ridgeway) |
| Julian Rozzell Jr. | Harlan |
| Bob Sorenson | Dale |
| Victor Verhaeghe | Ward Boss Fleming |
| Michael Warner | Klan Recruiter |
| James P. Anderson | The Judge
(uncredited) |
| Robert Bizik | Speakeasy Customer
(uncredited) |
| Kaydence Frank | Showgirl
(uncredited) |
| Frank Galdorise | Brothel Customer
(uncredited) |
| Deanna Gibson | Suffragette
(uncredited) |
| Nick Joseph | Bellhop
(uncredited) |
| Ashley Klein | Bordello Girl
(uncredited) |
| Dan Levinson | Musician
(uncredited) |
| Jerry Lobrow | Funeral Mourner
(uncredited) |
| Vincent Riviezzo | Mob Pallbearer
(uncredited) |
| Kent Sladyk | Gangster
(uncredited) |
| Sarah Sugrue | Young Girl
(uncredited) |